B2 modal verbs: deduction, obligation, advice and ability

Q: What modal verbs do you need for B2 First?

You need *can, could, may, might, must, should, would, have to, need to, ought to, had better* and *be able to*, and above all the perfect forms: *must have*, *can't have*, *might have*, *should have* and *needn't have*.

Q: What is the difference between mustn't and don't have to?

*Mustn't* means something is not allowed: *You mustn't use your phone in the exam.* *Don't have to* means something is not necessary: *You don't have to arrive before nine.*

Q: How do you use modals of deduction in Speaking?

In Speaking Part 2, use them to guess about the photos: *They must be at a wedding*, *She might be waiting for a friend*, *He can't be enjoying himself.* This shows the examiner that you can speculate, not just describe.

Q: What is the difference between could and was able to?

For general ability in the past, both work: *I could / was able to swim at five.* For one specific success, use *was able to* or *managed to*, not *could*. In the negative, *couldn't* is fine in both cases.

Q: Can I use may have and might have in the same way?

Yes. *May have*, *might have* and *could have* all express possibility about the past: *He may / might / could have missed the email.* *Could have* can also mean a past possibility that didn't happen: *You could have been hurt.*

Q: Are modal verbs tested in key word transformations?

Very often. Typical prompts are *I'm sure*, *It's possible that*, *It was a mistake to…* and *It wasn't necessary*, which become *must have*, *might have*, *shouldn't have* and *needn't have*.

B2 modal verbs go beyond can and must: you need modals of deduction about the past (must have, can't have, might have), the difference between don't have to, mustn't and needn't have, and criticism with should have. These forms are tested in Part 4 and used constantly in Speaking. This page belongs to our B2 grammar guide.

By Eleanor BennettLast updated: 13 October 2026

Modal verbs (can, could, may, might, must, should, will, would, plus have to, need to, ought to and be able to) add the speaker's attitude to a verb: how sure they are, how necessary something is, or whether it is a good idea.

At B1 you mostly use modals about the present and future. At B2 the key step is modal + have + past participle, which lets you talk about the past:

Deduction: She must have missed the bus (I'm sure), He can't have heard you (I'm sure he didn't), They might have got lost (it's possible).
Criticism and regret: You should have told me, I shouldn't have eaten so much.
Unnecessary past actions: You needn't have brought a present (but you did).

Modals are everywhere in the exam. In Speaking Part 2 you speculate about photos: They might be on a school trip; she must be feeling nervous. In key word transformations, a sentence with I'm sure, It's possible that or It wasn't necessary is often rewritten with a modal. In Writing, should and could help you make polite suggestions in a report or email.

Modals also appear inside other structures. In the result clause of conditionals you can use could or might instead of would, and the passive combines with modals as must be done or should have been done (see our passive voice guide).

Use it to…	Phrases
Deduction about the present	must (I'm sure it's true): The lights are on, so they must be at home. can't (I'm sure it's not true): That can't be Jan; he's in Berlin this week. might / may / could (possible): She might be stuck in traffic.
Deduction about the past: modal + have + past participle	must have: The ground is wet. It must have rained in the night. can't / couldn't have: He can't have finished already; he only started ten minutes ago. might / may / could have: I might have left my keys at work.
Obligation and necessity	must / have to (necessary): I have to wear a uniform at work. mustn't (prohibited): You mustn't park in front of the gate. don't have to / don't need to (not necessary): You don't have to come if you're busy.
Unnecessary past actions	needn't have + past participle (you did it, but it wasn't necessary): We needn't have booked; the restaurant was empty. didn't need to + infinitive (it wasn't necessary, usually so you didn't do it): I didn't need to take the bus because Dad gave me a lift.
Advice, criticism and regret	should / ought to: You should get that cough checked. should have / ought to have: You should have warned me it was so cold. had better (strong advice, often a warning): We'd better leave now or we'll miss the train.
Ability: past	could (general ability): I could ride a bike when I was four. was able to / managed to (one specific success): The fire spread quickly, but everyone was able to escape. couldn't (both general and specific): I couldn't open the jar.

Trap: You mustn't bring food; there will be lunch. German speakers are especially likely to make this mistake, because German muss nicht means "don't have to".
Fix: Mustn't means it is forbidden. For "not necessary", use don't have to or don't need to: You don't have to bring food.
Trap: He mustn't be at home; his car isn't there. Spanish no debe de estar and Italian non deve essere lead to mustn't for negative deduction.
Fix: The negative of deduction must is can't: He can't be at home.
Trap: She must forgot or She must to have forgotten.
Fix: Modals are followed by the bare infinitive. For the past, use have + past participle: She must have forgotten.
Trap: After three hours, we could reach the top. Using could for one specific achievement.
Fix: For a single success in the past, use managed to or was able to: we managed to reach the top.
Trap: You should had called me or You should of called me.
Fix: Should have + past participle: You should have called me. Should've is the spoken contraction; should of is never correct.
Trap: Confusing needn't have done and didn't need to do.
Fix: I needn't have cooked = I cooked, but it was unnecessary. I didn't need to cook = it wasn't necessary (and probably I didn't).

The task

Complete the second sentence so that it has a similar meaning to the first sentence, using the word given. Do not change the word given. You must use between two and five words, including the word given.

I'm sure Marek didn't read the instructions before he started.

CAN'T

Marek ______ the instructions before he started.

It wasn't necessary for you to clean the flat, but thank you.

NEED

You ______ the flat, but thank you.

1.
Item 1. I'm sure… didn't is a negative deduction about the past. The negative of deduction must is can't, and for the past we add have + past participle: can't have read.
2.
Count: can't = can + not (2 words), + have + read = 4 words. Within the limit. mustn't have read is wrong because mustn't is not used for deduction.
3.
Item 2. It wasn't necessary… but thank you tells us the person did clean the flat. An unnecessary action that happened is needn't have + past participle.
4.
Write needn't have cleaned: need + not + have + cleaned = 4 words. didn't need to clean is grammatically correct but does not show clearly that the cleaning happened, and with NEED as the key word the exam-style answer is needn't have cleaned.

Answer

Marek can't have read the instructions before he started. (4 words)

You needn't have cleaned the flat, but thank you. (4 words)

Practice · Reading & Use of English Part 4

Key word transformations 1: Mixed grammar

B2 modal verbs: frequently asked questions

What modal verbs do you need for B2 First?

You need can, could, may, might, must, should, would, have to, need to, ought to, had better and be able to, and above all the perfect forms: must have, can't have, might have, should have and needn't have.

What is the difference between mustn't and don't have to?

Mustn't means something is not allowed: You mustn't use your phone in the exam. Don't have to means something is not necessary: You don't have to arrive before nine.

How do you use modals of deduction in Speaking?

In Speaking Part 2, use them to guess about the photos: They must be at a wedding, She might be waiting for a friend, He can't be enjoying himself. This shows the examiner that you can speculate, not just describe.

What is the difference between could and was able to?

For general ability in the past, both work: I could / was able to swim at five. For one specific success, use was able to or managed to, not could. In the negative, couldn't is fine in both cases.

Can I use may have and might have in the same way?

Yes. May have, might have and could have all express possibility about the past: He may / might / could have missed the email. Could have can also mean a past possibility that didn't happen: You could have been hurt.

Are modal verbs tested in key word transformations?

Very often. Typical prompts are I'm sure, It's possible that, It was a mistake to… and It wasn't necessary, which become must have, might have, shouldn't have and needn't have.