Modal Verbs: 3 Rules to Avoid Common Mistakes

1. Introduction to Modal Verbs

Modal verbs are among the most frequently used words in English. They help us express ability, possibility, permission, obligation, advice, and more. However, because modal verbs behave differently from regular verbs, many learners make mistakes with them.

This lesson focuses on three simple rules that will help you avoid the most common errors:

1. Never change the modal verb.
2. Always use the base form of the verb after a modal.
3. Use “not” after the modal to make it negative.

By following these rules, you can use modal verbs correctly and confidently in speaking and writing.

Why Modal Verbs Matter

Reason	Explanation
Frequency	They appear constantly in daily conversation.
Meaning	They express important nuances like ability, obligation, and permission.
Exam success	IELTS, TOEFL, and GRE assess grammatical accuracy.
Professionalism	Correct use sounds polished and fluent.

Word Meaning Section

Word / Phrase	English Meaning	Bengali Meaning
Modal verb	a helping verb expressing ability, possibility, permission, etc.	মোডাল ক্রিয়া
Ability	the power or skill to do something	সক্ষমতা
Possibility	a chance that something may happen	সম্ভাবনা
Permission	approval to do something	অনুমতি
Obligation	something you must do	বাধ্যবাধকতা
Base form	the simplest form of a verb	ক্রিয়ার মূল রূপ
Negative	expressing “not”	নেতিবাচক

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2. What Are Modal Verbs?

Modal verbs are a special group of helping verbs. They modify the main verb and add meaning such as ability, permission, possibility, advice, or obligation.

Common Modal Verbs

Modal Verb	General Meaning
can	ability, possibility, permission
could	past ability, polite request, possibility
may	permission, possibility
might	possibility
should	advice, recommendation
ought to	advice, obligation
must	strong obligation, necessity
have to	obligation, necessity
will	future intention
shall	formal future, suggestion
would	polite request, hypothetical

8 Examples with Explanations

#	Example	Explanation
1	She can swim very well.	Ability.
2	Could you help me, please?	Polite request.
3	May I come in?	Asking permission.
4	It might rain later.	Possibility.
5	You should eat more vegetables.	Advice.
6	I must finish this report tonight.	Strong obligation.
7	We have to leave now.	Necessity.
8	I will call you tomorrow.	Future intention.
9	Shall I open the window?	Suggestion/offer.
10	Would you like some tea?	Polite offer.

Word Meaning Section

Word / Phrase	English Meaning	Bengali Meaning
Ability	skill or power	সক্ষমতা
Request	asking politely	অনুরোধ
Permission	approval	অনুমতি
Possibility	chance	সম্ভাবনা
Advice	recommendation	উপদেশ
Obligation	something necessary	বাধ্যবাধকতা
Necessity	something needed	প্রয়োজনীয়তা
Intention	plan	ইচ্ছা
Suggestion	idea	পরামর্শ
Offer	to give	প্রস্তাব

Note

Modal verbs do not take -s, -ed, or -ing endings. They are always followed by the base form of the main verb.

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3. Rule 1: Use the Modal Verb as It Is

The Rule

Never change a modal verb. It stays the same for all subjects—I, you, he, she, it, we, they—and in all tenses.

Correct vs. Incorrect

Correct	Incorrect
He can swim.	He cans swim.
She might go.	She mights go.
They should leave.	They shoulds leave.
It will rain.	It wills rain.
We could help.	We coulds help.

8 Examples with Explanations

#	Correct Sentence	Explanation
1	He can speak French.	No “s” on “can.”
2	She might join us.	“Might” stays the same.
3	They should arrive soon.	“Should” does not change.
4	I must finish now.	“Must” does not change.
5	We will meet at noon.	“Will” stays the same for all.
6	You could call him.	No extra ending.
7	He may attend the meeting.	“May” does not take “s.”
8	She would help if she could.	“Would” and “could” do not change.
9	It might snow tonight.	“Might” is invariable.
10	They shall follow the rules.	“Shall” remains the same.

Word Meaning Section

Word / Phrase	English Meaning	Bengali Meaning
Invariable	never changing	অপরিবর্তনীয়
Subject	the person or thing doing the action	কর্তা
Tense	time form of a verb	কাল
-s ending	third person singular marker	তৃতীয় পুরুষের “s”

Related Phrasal Verb

No phrasal verb is directly related, but note that modal verbs are often combined with main verbs that may themselves be part of a phrasal verb:

He can call back later.
She should give up smoking.

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4. Rule 2: Use the Base Form of the Verb

The Rule

After a modal verb, always use the base form of the main verb. Do not use “to” + verb (the full infinitive).

Correct vs. Incorrect

Correct	Incorrect
He might join us.	He might to join us.
You must finish your homework.	You must to finish your homework.
They can stay with us.	They can to stay with us.
She should not smoke.	She should not to smoke.
We would not arrive on time.	We would not to arrive on time.

8 Examples with Explanations

#	Correct Sentence	Explanation
1	He might join us.	Base verb “join.”
2	You must finish your homework.	“Finish” without “to.”
3	They can stay with us.	“Stay” without “to.”
4	She should not smoke.	“Smoke” without “to.”
5	We would not arrive on time.	“Arrive” without “to.”
6	I could help you.	“Help” is base form.
7	You may leave now.	“Leave” is base form.
8	He will call later.	“Call” is base form.
9	They must decide soon.	“Decide” is base form.
10	She can speak clearly.	“Speak” is base form.

Word Meaning Section

Word / Phrase	English Meaning	Bengali Meaning
Base form	the simplest verb form	ক্রিয়ার মূল রূপ
Infinitive	“to” + base verb	অসমাপিকা ক্রিয়া
Full infinitive	“to” + verb	পূর্ণ অসমাপিকা
Bare infinitive	base verb without “to”	অ-সমাপিকা ক্রিয়া (to ছাড়া)
Main verb	the verb carrying the main meaning	প্রধান ক্রিয়া

Related Phrasal Verb

Modal + base verb can be combined with phrasal verbs:

Modal + Phrasal Verb	Example
can call back	I can call back later.
should give up	You should give up smoking.
must look after	She must look after her brother.
could turn off	Could you turn off the light?

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5. Rule 3: Form Negatives with “Not”

The Rule

To make a modal verb negative, simply add “not” after the modal. Do not use “don’t,” “doesn’t,” “isn’t,” “aren’t,” “wasn’t,” or “won’t” with most modals.

Correct vs. Incorrect

Correct	Incorrect
You should not smoke.	You don’t should smoke.
We could not call you.	We not could call you.
She cannot drive.	She doesn’t can drive.
They will not attend.	They won’t not attend. (double negative)
He may not come.	He doesn’t may come.

8 Examples with Explanations

#	Correct Sentence	Explanation
1	You should not smoke.	“Not” after “should.”
2	We could not call you.	“Not” after “could.”
3	She cannot drive.	One word: cannot.
4	They will not attend.	“Not” after “will.”
5	He may not come.	“Not” after “may.”
6	I might not go.	“Not” after “might.”
7	You must not lie.	“Not” after “must.”
8	She would not agree.	“Not” after “would.”
9	We shall not forget.	“Not” after “shall.”
10	He ought not to smoke.	“Not” after “ought” before “to.”

Word Meaning Section

Word / Phrase	English Meaning	Bengali Meaning
Negative	expressing “not”	নেতিবাচক
“Not”	used to make negative	না
Contraction	shortened form	সংক্ষিপ্ত রূপ
Cannot	can not (one word)	পারি না
Ought not to	should not	উচিত না
Double negative	incorrect use of two negatives	দ্বৈত নেতিবাচক

Related Phrasal Verb

No phrasal verb directly, but note that after the negative modal, a phrasal verb can follow:

He should not give up.
They could not turn down the offer.

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6. Special Cases: “Have to” and “Cannot”

“Have to” Is Different

Unlike most modal verbs, “have to” does not behave exactly like other modals in negatives and questions.

• Negative: don’t have to / doesn’t have to
• Question: Do/Does + subject + have to + base verb?

This is because “have to” is a semi-modal; it uses “do” as a helping verb in negatives and questions.

8 Examples with Explanations

#	Example	Explanation
1	I have to work tomorrow.	Obligation.
2	She has to leave early.	Third person singular uses “has to.”
3	You don’t have to come.	Negative uses “don’t.”
4	He doesn’t have to pay.	Negative uses “doesn’t.”
5	Do you have to go now?	Question uses “Do.”
6	Does she have to attend?	Question uses “Does.”
7	We had to cancel.	Past form “had to.”
8	They didn’t have to wait.	Past negative.
9	Did you have to work late?	Past question.
10	I will have to leave early.	Future with “will have to.”

“Cannot” Is One Word

In standard written English, “cannot” is usually written as one word. The contraction “can’t” is common in informal speech and writing.

Full Form	Contraction
cannot	can’t

8 Examples with Explanations

#	Sentence	Explanation
1	I cannot attend the meeting.	One word “cannot.”
2	She can’t come today.	Contraction.
3	You cannot park here.	Prohibition.
4	He can’t swim.	Lack of ability.
5	We cannot afford that.	Not possible.
6	They can’t hear you.	Inability.
7	It cannot be true.	Impossibility.
8	I can’t remember his name.	Informal negative.

Word Meaning Section

Word / Phrase	English Meaning	Bengali Meaning
Semi-modal	a verb with modal and regular features	আধা-মোডাল
Have to	must; necessity	করতে হবে
Don’t have to	not necessary	প্রয়োজন নেই
Cannot / can’t	not able; not allowed	পারা যায় না; নিষেধ
Prohibition	not allowed	নিষেধ
Impossibility	cannot happen	অসম্ভবতা
Contraction	shortened form	সংক্ষিপ্ত রূপ

Related Phrasal Verb

Phrasal Verb	Meaning	Example
Have to do with	be related to	This has to do with your behavior.
Have got to	informal “must”	I’ve got to go now.

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7. Phrasal and Idiomatic Modal Expressions

Some modal-like expressions include phrasal or idiomatic forms.

8 Examples with Explanations

#	Expression	Meaning	Example
1	have got to	must (informal)	I’ve got to leave now.
2	ought to	should	You ought to apologize.
3	be able to	can (ability)	She is able to solve it.
4	be supposed to	should; expected	He’s supposed to arrive at 9.
5	be allowed to	may; permitted	Are we allowed to enter?
6	had better	should (with warning)	You’d better hurry.
7	would rather	prefer	I’d rather stay home.
8	used to	past habit	I used to smoke.
9	be likely to	will probably	It’s likely to rain.
10	be about to	very soon	The show is about to start.

Word Meaning Section

Word / Phrase	English Meaning	Bengali Meaning
Have got to	must	অবশ্যই করতে হবে
Ought to	should	উচিত
Be able to	can	সক্ষম হওয়া
Be supposed to	expected	প্রত্যাশিত
Be allowed to	permitted	অনুমোদিত
Had better	should with warning	বরং ভালো হবে
Would rather	prefer	বরং চাই
Used to	past habit	অভ্যস্ত ছিল
Be likely to	probable	সম্ভাব্য
Be about to	soon	প্রায়

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8. Grammar Focus: Modal Verb Structures and WH Questions

Basic Sentence Structure

Subject + modal verb + base verb + (object/complement)

Example
She can speak English.
You should rest.
They must leave.

Negative Structure

Subject + modal verb + not + base verb

Example
She cannot speak English.
You should not rest.
They must not leave.

Interrogative Structure

Modal verb + subject + base verb?

Example
Can she speak English?
Should you rest?
Must they leave?

WH Question Structure

WH word + modal verb + subject + base verb?

WH Word	Question	Answer
What	What should I do?	You should rest.
When	When can we leave?	We can leave at 5.
Where	Where must they go?	They must go home.
Why	Why should she attend?	She should attend to learn.
How	How could he help?	He could help with the report.

Affirmative, Negative, and Interrogative Forms

Form	Example
Affirmative	He can swim.
Negative	He cannot swim.
Interrogative	Can he swim?
Affirmative	You should rest.
Negative	You should not rest.
Interrogative	Should you rest?
Affirmative	They must leave.
Negative	They must not leave.
Interrogative	Must they leave?

5 WH-Question Examples with Answers

#	WH Question	Possible Answer
1	What should I do?	You should talk to your teacher.
2	When can we leave?	We can leave at 6:00.
3	Why must she attend?	She must attend because it’s important.
4	How could he help?	He could help by giving advice.
5	Where may we sit?	You may sit anywhere you like.

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9. Common Errors and How to Avoid Them

Error 1: Adding -s to Modal

Incorrect	Correct
He cans swim.	He can swim.
She mights go.	She might go.
They shoulds leave.	They should leave.
It wills rain.	It will rain.

Error 2: Using “to” After Modal

Incorrect	Correct
He might to join us.	He might join us.
You must to finish.	You must finish.
They can to stay.	They can stay.
She should not to smoke.	She should not smoke.

Error 3: Wrong Negative Form

Incorrect	Correct
I don’t can drive.	I cannot drive. / I can’t drive.
You don’t should smoke.	You should not smoke.
They doesn’t may come.	They may not come.

Error 4: Misplacing “Not”

Incorrect	Correct
We not could call you.	We could not call you.
He not might come.	He might not come.
She not should go.	She should not go.

Error 5: Using “Cannot” as Two Words in Formal Writing

Less Formal	More Formal
can not (two words)	cannot (one word)

Error 6: Treating “Have to” Like Other Modals in Negative

Incorrect	Correct
I haven’t to go.	I don’t have to go.
She hasn’t to work.	She doesn’t have to work.

Error 7: Forgetting the Base Verb After Modal

Incorrect	Correct
He can swim. (already correct)	–
She should to rest. (already covered)	–
I must go. (correct)	–

Error 8: Using “Don’t” Instead of “Not” with Modals

Incorrect	Correct
You don’t should smoke.	You should not smoke.
I don’t can drive.	I cannot drive.

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10. Special Tips for Learners

Tip 1: Memorize the Three Rules

1. Never change the modal.
2. Use the base verb.
3. Add “not” after the modal.

Tip 2: Think of “Cannot” as One Word

In formal writing, use cannot.

Tip 3: Treat “Have to” Differently

For negatives and questions, use do/does/did.

Tip 4: Practice with All Pronouns

Say each modal with I, he, she, it, we, you, they to see that it never changes.

Tip 5: Use Contractions in Speaking

• cannot → can’t
• should not → shouldn’t
• will not → won’t

Tip 6: Listen for Modal Patterns

Notice how native speakers use modals without “to” and with “not.”

Tip 7: Avoid Double “to”

Never say “must to,” “should to,” “can to.”

Tip 8: Write Sentences in All Forms

For each modal, write affirmative, negative, and question.

Tip 9: Be Careful with “Ought to”

“Ought” is always followed by “to”:

You ought to rest.

Tip 10: Review Regularly

Practice a few modal sentences daily until they feel automatic.

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11. Tips for Competitive Exams: IELTS, TOEFL, GRE

IELTS

Skill	How It Is Tested	Strategy
Writing Task 2	Giving opinions and recommendations	Use should, must, can appropriately.
Speaking	Expressing ability, advice, obligation	Use modal verbs accurately.
Listening	Understanding modal meanings	Recognize shades of obligation/possibility.
Grammar Accuracy	Errors with modals lower band	Avoid “to” after modals; use “not” correctly.

TOEFL

Skill	How It Is Tested	Strategy
Writing	Academic essays	Use modals for hedging: may, might, could.
Speaking	Integrated tasks	Use must/have to for obligation.
Listening	Lectures with modal verbs	Note precise meaning.

GRE

Skill	How It Is Tested	Strategy
Analytical Writing	Formal argumentation	Use should, must, may to qualify claims.
Text Completion	Modal meanings in context	Understand nuance of each modal.

Exam Rules

• Never add “to” after a modal verb.
• Use base form after modals.
• Use “not” after modals for negation.
• Treat “have to” as semi-modal (use do/does in negative/questions).
• Use “cannot” as one word in formal writing.
• Practice modal usage in timed essays.

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12. Practice Exercises

Exercise 1: Fill in the Blanks

Choose the correct form to complete each sentence.

1. He ______ swim very well. (can / cans)
2. She ______ to finish the report. (must / must to)
3. They ______ not come to the party. (may / might)
4. I ______ drive a truck. (cannot / don’t can)
5. You ______ to see a doctor. (should / ought)
6. We ______ not leave yet. (need / should)
7. ______ you help me, please? (Can / Do can)
8. She ______ not go out tonight. (will / is)
9. They ______ attend the meeting tomorrow. (have to / has to)
10. He ______ not smoke here. (must / does)

Exercise 2: Multiple Choice Questions

Choose the correct sentence.

1. Which sentence is correct?
   a) She cans swim.
   b) She can swim.
   c) She can to swim.
   d) She can swimming.
2. Which sentence is correct?
   a) You must to finish.
   b) You must finish.
   c) You must finishing.
   d) You must to finishing.
3. Which sentence is correct?
   a) I don’t can drive.
   b) I cannot drive.
   c) I can’t to drive.
   d) I don’t cannot drive.
4. Which sentence is correct?
   a) They mights come.
   b) They might to come.
   c) They might come.
   d) They might coming.
5. Which sentence is correct?
   a) You should not to smoke.
   b) You don’t should smoke.
   c) You should not smoke.
   d) You not should smoke.
6. Which sentence is correct?
   a) We not could call you.
   b) We could not call you.
   c) We could not to call you.
   d) We don’t could call you.
7. Which is the correct negative of “I can go”?
   a) I can not go. (also acceptable, but less common)
   b) I cannot go.
   c) I don’t can go.
   d) I can’t to go.
8. Which sentence is correct?
   a) He will arrives soon.
   b) He will arrive soon.
   c) He wills arrive soon.
   d) He will to arrive soon.
9. Which sentence is correct for “have to”?
   a) I haven’t to go.
   b) I don’t have to go.
   c) I doesn’t have to go.
   d) I don’t has to go.
10. Which question is correct?
    a) Do you can swim?
    b) Can you swim?
    c) Can you to swim?
    d) Do you can to swim?

Exercise 3: Sentence Transformation / Error Correction

Rewrite each sentence correctly.

1. He cans swim very fast.
2. She might to join us for dinner.
3. I don’t can drive a car.
4. You should not to smoke here.
5. We not could call you last night.
6. They mights arrive late.
7. He must to finish his work.
8. She doesn’t may attend the meeting.
9. We will not to arrive on time.
10. He oughts to study more.

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13. Answer Key with Explanations

Exercise 1 Answers

1. can
   Explanation: Modal verb never changes.
2. must
   Explanation: “must” + base verb without “to.”
3. may / might
   Explanation: Both possible; no change.
4. cannot
   Explanation: One word negative.
5. ought
   Explanation: “ought” + “to” is correct.
6. should
   Explanation: “should not” + base verb.
7. Can
   Explanation: Question form “Can you…”
8. will
   Explanation: “will not” for future negative.
9. have to
   Explanation: “They have to” (plural subject).
10. must
    Explanation: “must not” for prohibition.

Exercise 2 Answers

1. b) She can swim.
   Explanation: No “s,” no “to.”
2. b) You must finish.
   Explanation: Base verb after must.
3. b) I cannot drive.
   Explanation: One word negative.
4. c) They might come.
   Explanation: No “s,” no “to.”
5. c) You should not smoke.
   Explanation: “not” after modal.
6. b) We could not call you.
   Explanation: “not” after modal.
7. b) I cannot go.
   Explanation: One word “cannot.”
8. b) He will arrive soon.
   Explanation: No “s,” no “to.”
9. b) I don’t have to go.
   Explanation: “have to” uses “do” in negative.
10. b) Can you swim?
    Explanation: Modal first in question.

Exercise 3 Answers

1. He can swim very fast.
   Explanation: Remove “s” from “can.”
2. She might join us for dinner.
   Explanation: Remove “to.”
3. I cannot drive a car.
   Explanation: Use “cannot” or “can’t.”
4. You should not smoke here.
   Explanation: Remove “to.”
5. We could not call you last night.
   Explanation: Place “not” after “could.”
6. They might arrive late.
   Explanation: Remove “s.”
7. He must finish his work.
   Explanation: Remove “to.”
8. She may not attend the meeting.
   Explanation: Use “may not” not “doesn’t may.”
9. We will not arrive on time.
   Explanation: Remove “to.”
10. He ought to study more.
    Explanation: “ought” + “to” + base verb.

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14. Conclusion

Modal verbs are essential for expressing ability, possibility, permission, obligation, and advice. By following three simple rules—never change the modal, use the base verb, and add “not” after the modal—you can avoid the most common errors.

Remember these key points:

• Can, could, may, might, should, must, will, would, shall never change form.
• Always use the base form of the verb after a modal.
• Form the negative with not after the modal.
• Use cannot as one word.
• Treat have to differently: use do/does/did in negatives and questions.

By practicing these rules and completing the exercises, you will use modal verbs with confidence. This will improve both your spoken and written English, especially in academic and professional contexts, and help you succeed in exams like IELTS, TOEFL, and GRE.

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Call to Action: Write five sentences using different modal verbs in the affirmative form. Then, change each sentence into its negative and question forms. Practice saying them aloud. Finally, review your next essay or email and check for any modal verb mistakes—especially “to” after modals and missing “not” in negatives. Keep practicing daily until these rules become automatic.