Must, May and Might: Meaning, Uses and Examples in English

Overview

The modal verbs must, may, and might each carry meanings that sit at different points on a scale between certainty and possibility. Must sits at the stronger end: it signals either an obligation that cannot be avoided or a logical conclusion the speaker is highly confident about. May and might operate in less certain territory, expressing what is possible or permitted rather than what is required or definite.

Knowing which one to choose requires understanding not just the grammar but the degree of confidence or authority the speaker intends to convey.

The Form of Must, May, and Might

All three verbs follow the standard modal pattern. No subject causes any of them to change form, no to appears before the following verb, and the main verb stays in its base form throughout.

Must has no direct past tense form of its own. To express past obligation, had to is used instead. To express a past deduction, must have followed by a past participle is used.

May and might also have no past obligation or permission form. For past possibility or deduction, may have and might have followed by a past participle are the correct forms.

Negatives are formed by placing not directly after the modal. The contracted forms are mustn't and mightn't. May not is typically written as two words, particularly in formal contexts.

Questions with may and might are formed by inversion, though might questions are rare in everyday speech. Must questions are possible but tend to sound formal or emphatic; do I have to is more natural in conversational queries about obligation.

Must for Obligation

The most familiar use of must is to express strong obligation or necessity. This obligation comes from the speaker or from an authority the speaker represents. It is direct, serious in tone, and leaves little room for negotiation.

The strength of must sets it apart from should. While should recommends, must requires. When expressing past obligation, must gives way to had to, because must has no past tense form in this function.

Must for Logical Deduction

Must also expresses a logical deduction: a conclusion the speaker reaches based on available evidence. This use does not express obligation at all; it expresses near certainty about a present or past situation.

The structure must have followed by a past participle applies this deductive meaning to past events.

Mustn't vs. Don't Have To

A critical distinction at this level is the difference between mustn't and don't have to. These two negative forms do not mean the same thing, and confusing them produces statements that are either misleading or incorrect.

Mustn't means the action is forbidden.

Don't have to means the action is not required. It is not prohibited; doing it remains an option.

May for Permission

In formal contexts, may is the conventional modal for asking or granting permission. It sounds more measured and respectful than can in situations that call for formality, such as addressing someone in authority or speaking in an official or professional setting.

May not in this context means permission is refused or withheld.

May and Might for Possibility

Both may and might express possibility. The difference between them is a matter of degree. May suggests a stronger possibility, closer to fifty percent or above. Might suggests a weaker possibility, something the speaker considers less likely or more uncertain.

In everyday informal English, many speakers use may and might interchangeably for present and future possibility. In careful or formal writing, the distinction is worth observing.

For past possibility, both may have and might have followed by a past participle express the idea that something was possibly the case, but the speaker is not certain.

Comparing Must, May, and Might for Deduction and Possibility

The negative of a deductive must is not mustn't. When a speaker is nearly certain something is not the case, the correct form is can't or couldn't. Mustn't belongs to obligation, not deduction.

Common Mistakes to Avoid

Mistake 1: Using Must for Past Obligation Instead of Had To

Must has no past tense form for obligation. Attempting to use must to describe a past requirement produces a grammatically incorrect sentence.

Mistake 2: Confusing Mustn't and Don't Have To

Mustn't forbids the action. Don't have to simply removes the requirement. Using one where the other is meant creates a serious miscommunication.

Mistake 3: Using Mustn't for a Negative Deduction

When a speaker is almost certain that something is not true, the correct modal is can't or couldn't, not mustn't. Mustn't signals prohibition, not logical impossibility.

Mistake 4: Adding To After Must, May, or Might

Like all modal verbs, none of these three takes to before the following main verb.

Mistake 5: Using May Have or Might Have Without the Past Participle

The past forms may have and might have must be followed by a past participle. Using a base form instead produces an ungrammatical structure.

Mistake 6: Using Might Instead of May in Formal Permission Contexts

In formal settings, asking for permission with might sounds unusual in modern English. May is the conventional choice.

Practice Exercises

Exercise 1: Choose the Correct Modal

Choose must, may, or might to complete each sentence.

1. You ___ not enter this area without authorisation.
2. She hasn't replied yet. She ___ not have seen the message.
3. The lights are still on in his office. He ___ still be working.
4. Take an umbrella. It ___ rain later, though it's not certain.
5. All applicants ___ provide two forms of identification.
6. She ___ be at the airport by now. Her flight leaves in an hour.

Exercise 2: Mustn't or Don't Have To?

Choose mustn't or don't have to to complete each sentence correctly.

1. You ___ tell anyone about this. It's confidential.
2. You ___ wear a suit to the meeting. Smart casual is fine.
3. Visitors ___ touch the exhibits in this gallery.
4. You ___ finish everything on your plate if you're full.
5. Drivers ___ use their phones while operating a vehicle.

Exercise 3: Rewrite Using the Past Form

Rewrite each sentence using the correct past form: had to, must have, may have, or might have.

1. She is very knowledgeable. She studied this subject for years. (deduction)
2. He is not at his desk. Perhaps he went to lunch. (possibility, less certain)
3. The office was closed. They were required to reschedule. (past obligation)
4. The door is open. Someone left it unlocked. (near-certain deduction)
5. I am not sure, but perhaps she caught an earlier train. (possibility)

Exercise 4: Correct the Mistake

Each sentence contains one error. Rewrite it correctly.

1. He must arrive late last night because of the delays.
2. That mustn't be the correct file. The date is completely wrong.
3. You must to show your ticket at the gate before boarding.
4. She might have forget to charge her phone before the trip.
5. You mustn't book in advance; it's optional, but recommended.

Summary

Choosing accurately between must, may, and might comes down to how certain the speaker is and whether the sentence expresses obligation, permission, or possibility.