Mastering Modals of Permission and Obligation: The Ultimate English Guide

Do you know how to use can, must, have to, and mustn’t correctly? Expressing what is allowed, required, or prohibited is a fundamental part of daily English communication. However, choosing the wrong modal verb can make you sound unintentionally rude or completely change the meaning of your sentence.

In this comprehensive guide, we will break down the rules of permission, prohibition, obligation, and lack of obligation. With clear formulas and real-world examples, you will learn to navigate these structures smoothly.

Quick Comparison: Look at These Examples

Notice how the meaning shifts dynamically depending on the modal verb used:

• 🧥 Permission: You can put your shoes and coat over there.
• 🚲 Prohibition: You can’t leave your bike there.
• 🚗 External Obligation: You have to have a license to drive a car.
• 🚴‍♂️ No Obligation: You don’t have to have a license to cycle on the roads.

1. Asking for and Giving Permission

When you want to ask if an action is allowed, or if you want to grant permission to someone else, you have three primary options depending on the level of formality.

🔹 Can (Informal & Neutral)

This is the most common way to ask for and give permission in everyday conversations with friends, family, and colleagues.

• Can I sit here?
• You can use my car if you like.
• Can I make a quick suggestion?

🔹 Could (Polite & Formal)

We use could to ask for permission politely. Note: You can only use could to ask for permission, never to give it.

• Could I ask you a personal question?
• Could I interrupt you for just a brief moment, please?
• Could I borrow your pen, please? (To answer yes, you must switch back to can: “Yes, you can.”)

🔹 May (Very Formal & Official)

May is the most formal way to request or grant permission. It is common in professional environments, official documents, or signs.

• May I see your passport and boarding pass, please?
• Customers may request a full refund within a period of 30 days.
• These textbook pages may be photocopied for classroom use.

2. Prohibition: Saying What Is Not Allowed

When an action is strictly against the rules or forbidden, we use can’t or mustnot / mustn’t. While they both mean “no,” they are used in slightly different contexts.

🛑 Can’t (Rules Outside of Our Control)

We use can’t when something is against the rules or laws, especially when we did not create the rules ourselves.

• What does this sign say? Oh, we can’t park here.
• You can’t take photos inside the museum; they are incredibly strict about it.
• I’m sorry, but we can’t sell knives to anyone under 18.

🛑 Must Not / Mustn’t (Strict Prohibitions & Personal Rules)

Must not is highly authoritative. It is frequently seen on public signs and official notices. The contracted form, mustn’t, is used when the speaker is personally enforcing the rule.

• Official Signs: Visitors must not park in the staff courtyard. Baggage must not be left unattended.
• Speaker Authority (Parent to child): You mustn’t say things like that to your sister!
• Speaker Authority (Teacher to student): You mustn’t be late to class tomorrow.
• Personal Rule: I mustn’t let that mistake happen again.

3. Obligation: Saying What Is Required

To express that an action is necessary or required, we use have to and must. This is one of the most common points of confusion for English learners, but the distinction is simple once you know the secret: Internal vs. External Obligation.

[Have to] ───> Driven by External Rules (Laws, Work, School)
[Must]    ───> Driven by Internal Feelings (Speaker's Choice)

💼 Have to (External Obligation)

Use have to when the requirement comes from an outside force, such as a law, your boss, a school rule, or a scheduled event. You have no personal choice in the matter.

• We have to wear a uniform when we are working at the front reception desk.
• When do we have to hand in our final assignments?
• Al has to work tomorrow morning, so he can’t come to the party tonight.

🧠 Must (Internal Obligation / Strong Necessity)

Use must when the obligation comes directly from the speaker’s own feelings, beliefs, or personal sense of duty.

• I must phone my dad tonight; it’s his 60th birthday! (This is my personal choice).
• You must hand in your homework on Tuesday, or you will lose 10% of your mark. (An authority figure stating a direct rule).
• Seat belts must be worn by all passengers at all times.

⚠️ Past Tense Alert: You cannot use “must” to express obligation in the past. To say that something was required yesterday or last week, you must use had to.

• ❌ Incorrect: I must pay a fine last week.
• Correct: I had to pay a $50 fine to renew my passport last week.

4. No Obligation: Saying What Is Optional

One of the biggest traps in English grammar is assuming that mustn’t and don’t have to mean the same thing. They don’t! While mustn’t means “do not do it because it is forbidden,” don’t have to simply means “it is optional.”

🟢 Don’t Have to (Completely Optional)

You can do the action if you want to, but it is not compulsory. There is zero penalty if you skip it.

• You don’t have to wear a tie in our office, but some people still like to dress formally.
• You don’t have to physically go to the bank to complete a wire transfer; you can do it easily online.
• You don’t have to come with me to the grocery store, honestly. I’ll be completely fine on my own!

🗺️ Master Summary Cheat Sheet

Category	Modal Verb	Formality / Context	Quick Example
Permission	Can	Informal / Neutral	“You can borrow my jacket.”
	Could	Polite Request Only	“Could I leave early?”
	May	Very Formal / Official	“You may begin the exam.”
Prohibition	Can’t	Against general rules	“We can’t smoke inside.”
	Mustn’t	Strict / Speaker’s authority	“You mustn’t touch that.”
Obligation	Have to	External rules / laws	“I have to pay taxes.”
	Must	Internal choice / Past form is had to	“I must exercise more.”
No Obligation	Don’t have to	Optional / Not required	“You don’t have to pay.”

🧠 Test Your Skills Interactively!

Do you want to check if you can use permission and obligation modals like a native speaker? Scroll straight to the top of our interactive grammar dashboard at Advanced English Lab! Take Grammar Test 1 to practice with real-time exercises, track your progress, and lock down these grammar rules once and for all.