Modals of Deduction: Must, Might, Can't

What are modals of deduction?

A deduction is a conclusion you make from the information you have. You are not completely sure, but you have a reason for your guess. We use the modal verbs must, may, might, could and can't to show how sure we are.

The lights are off and nobody is answering. They must be out.

The basic idea is introduced on the Modal Verbs page. This page goes further: continuous forms, past deduction and the most common mistakes.

The certainty scale

Notice two things:

• The opposite of must for deduction is can't, not mustn't.
• Could is used for possibility in positive sentences, but couldn't means you are sure something is impossible. For a negative possibility, use may not or might not.

May and might mean almost the same here. Some speakers feel might is a little less sure, but the difference is very small.

How to form modals of deduction about the present

Modal + base verb

Use this for states and general situations now.

Modal + be + -ing

Use this for actions happening right now.

How to form modals of deduction about the past

Use modal + have + past participle (the third form of the verb: gone, seen, left). Have never changes to has or had here.

Past continuous deduction

Use modal + have been + -ing for an action in progress at a past time.

• He didn't hear the doorbell. He must have been sleeping.
• I'm not sure why she was late. She might have been sitting in traffic.

When to use modals of deduction

1. Must: you are almost certain it is true

You have strong evidence.

• You've been travelling all day. You must be exhausted.
• There's a long line outside. The new café must be popular.
• Her car isn't in the driveway. She must have left already.

2. May, might, could: it is possible

You have some evidence, but there are other possibilities.

• Take an umbrella. It might rain later.
• I can't find my keys. I could have left them at work.
• Don't call now. They may be having dinner.

3. Can't and couldn't: you are almost certain it is not true

The evidence makes it seem impossible.

• That can't be Anna. She's in Montreal this week.
• He can't have read the email. He replied with the wrong date.
• They couldn't have walked here so fast. They must have driven.

For past deduction, can't have and couldn't have are both correct and mean the same.

4. Asking about possibility

In questions, use could or might, or can with words like who, what or where.

• Who could that be at the door?
• Could they have got lost?
• Where can she be? She left an hour ago.

Can't vs mustn't

These two look like opposites of must, but only one works for deduction.

In North American English, people sometimes use must not (the full form) for a negative deduction: He must not be home. It means I think he probably isn't home. Can't is stronger and works in every variety of English, so it is the safer choice.

Could have: deduction or missed chance?

Could have + past participle has two meanings. The context tells you which one.

• Deduction: Where's Tom? He could have gone to the gym. (Maybe that is what happened.)
• Missed chance: You could have told me! (It was possible, but you didn't.)

The second meaning, with should have and needn't have, is explained on Advanced Past Modals.

Common mistakes

Mistake 1: Mustn't for negative deduction

• Wrong: She mustn't be at work. It's Sunday.
• Correct: She can't be at work. It's Sunday.
• Reason: mustn't means not allowed.

Mistake 2: Couldn't for a negative possibility

• Wrong: I'm not sure, but he couldn't be at home.
• Correct: I'm not sure, but he might not be at home.
• Reason: couldn't means you are sure; for maybe not, use might not or may not.

Mistake 3: Can for a guess about one situation

• Wrong: Someone's at the door. It can be the delivery driver.
• Correct: It could be / might be the delivery driver.
• Reason: positive can is for general possibility (It can get very cold here in January), not a guess about now.

Mistake 4: Wrong form after the modal

• Wrong: She must has left. / She must had left.
• Correct: She must have left.
• Reason: after a modal, always have + past participle.

Mistake 5: Past simple instead of past participle

• Wrong: They might have went home.
• Correct: They might have gone home.
• Reason: use the third form (gone), not the past simple (went).

Mistake 6: Adding to after the modal

• Wrong: It must to be expensive.
• Correct: It must be expensive.
• Reason: modals are followed by the base verb with no to.

Quick reference

Common questions

What is the opposite of 'must' for deduction?

Use can't (or couldn't): It must be true → It can't be true. Mustn't is about rules, not about how sure you are.

What is the difference between 'must have' and 'had to'?

Must have done is a deduction about the past: He must have left early (I'm sure he did). Had to is a past obligation: He had to leave early (it was necessary).

Can I use 'has to be' for deduction?

Yes. Has to be and have to be can be used like must, especially in speech: This has to be the right address. It sounds very sure.

Practise next

• Modal Verbs — all the main uses of modal verbs
• Advanced Past Modals — needn't have, should have and missed chances
• Hedging Language — use may and might to sound less direct
• Modal Verbs Quiz (B1) — check the basics