Modal verbs: Permission, obligation, prohibition, necessity

Grammar / B2 Grammar lessons and exercises / Modal verbs: Permission, obligation, prohibition, necessity

✏️ Exercises

Permission

The most common modal verbs to talk about permission are can, could, may, and might.

Asking for Permission

  • Informal: Can I sit here?
  • Formal: Could I use this chair? / May I use this chair? / Might I use this chair?

Giving Permission

  • Informal: You can sit here.
  • Formal: You may use this chair.

Refusing Permission

  • Informal: Sorry, you can’t use this chair.
  • Formal: Sorry, you may not use this chair.

Permission in the Past

To talk about permission in the past, we use was/were allowed to or the negative couldn’t. ❗ We do not use the affirmative could for past permission.

  • He wasn’t allowed to sit down during the lesson.
  • He couldn’t sit down during the lesson.
  • He was allowed to receive visits.
  • ✅ He could receive visits. → This is acceptable in some contexts (especially British English), but generally avoided in formal grammar teaching.

Obligation

Illustrative chart explaining the difference in meaning between 'must' for personal obligation and 'have to' for external obligation, with 'mustn't' for prohibition and 'don't have to' for lack of necessity, including examples and pictograms of a doctor and a patient.

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Must / Have to

Both express obligation, but the source differs:

  • I must give up smoking. → Personal decision (speaker’s own rule).
  • I have to give up smoking. → External rule (doctor, law, etc.).

In informal English, we often use ’ve got to:

  • I’ve got to be there before ten.

In the past, we always use had to:

  • I had to give up smoking. (personal or external)

Mustn’t / Don’t have to

These negatives have very different meanings:

  • You mustn’t reveal the source. → Prohibition (it’s forbidden).
  • You don’t have to arrive before 7. → No obligation (it’s optional).

Necessity

Grammar chart explaining 'needn't', 'don't need to', 'didn't need to', and 'needn't have' with examples showing when something was unnecessary and whether it was done or not.

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Need to / Don’t need to

Used to say something is (or isn’t) necessary:

  • We need to/have to confirm our reservations before Friday.
  • You don’t need to/don’t have to believe in God to be good.

Don’t need to vs. Needn’t

  • General necessity: I don’t need to wear glasses. (always)
  • Specific occasion: You don’t need to / needn’t wash the dishes. (just now)

Didn’t need to vs. Needn’t have

  • You did it, but it wasn’t necessary:You didn’t need to buy / needn’t have bought anything. ✅
  • You didn’t do it, and it wasn’t necessary:I didn’t need to buy anything. ✅ ❌ I needn’t have bought anything. → (implies you DID buy it!)

Other Expressions: Permission, Possibility & Obligation

Grammar chart titled 'Permission, Possibility, & Obligation' with examples and rules for using expressions like 'be able to,' 'be allowed to,' 'be permitted to,' 'be supposed to,' 'be meant to,' and 'had better.'

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Be able to / Be allowed to

Used instead of can/could for permission or ability, especially in tenses where can isn’t available.

  • We were allowed to eat all we wanted.
  • You won’t be able to finish on time.

💡 We do not say: “It is allowed to...”✅ Say: “You are allowed to...” or “Using phones is allowed.”

Be (not) permitted to

Formal way to state rules or laws. Both personal and impersonal forms are possible.

  • It is not permitted to take photos.
  • Picnics are not permitted in the park.
  • Employees are permitted to use mobile devices.

Be supposed to / Be meant to

Express expected behavior, rules, or plans.

  • We are supposed/meant to check in one hour before take-off.
  • You aren’t supposed/meant to be here!

Had better / Had better not

Used for strong advice or warnings (often with negative consequences). Always use ’d better (not “hadn’t better”).

  • We’d better hurry up or we’ll miss our train.
  • You’d better not tell her you broke the vase—she’ll get very mad.
  • ❌ You hadn’t better tell her...