Past Modals: Should Have, Must Have, Might Have

All of these share one shape — modal + have + past participle — but they do two different jobs. Should have often expresses regret or criticism: the thing didn't happen, and you wish it had. Must have, can't have and might have express deduction or possibility: you're saying how sure you are about the past. Keep those jobs apart and the form becomes easier to manage.

Should have: the thing you didn't do

In regrets and criticism, should have normally means it didn't happen. The same form can target your own choice or someone else's.

Because it carries criticism, You should have… can sting. If you only want to suggest a better option, It might have been easier to… softens it a lot.

Should have can also describe an expected result: You should have received the email by now. Here the speaker expects that it probably arrived; the sentence does not say it failed to arrive.

Guessing about the past

Three levels of confidence. These are deductions from evidence — you weren't there, so you're reasoning about what happened.

Note the negative pair. The opposite of must have is can't have, not mustn't have — mustn't belongs to prohibition, a different meaning entirely.

The one that means two things

Could have and might have do double duty. Besides "maybe it happened", they also describe a possibility that never came true — close to a third conditional.

Context does the sorting, and native speakers rely on it without noticing. In writing, if the sentence could be read either way and the difference matters, a small addition fixes it: You could easily have told me reads clearly as reproach.

Common mistakes

"Should of." It sounds identical to should've in speech, which is exactly why it gets written — but of is a preposition and can't sit here.

A past simple instead of a participle. After have, irregular verbs take their third form.

Using must have for past obligation. Must have is a guess. For something you were required to do, use had to.

"Mustn't have" for a negative guess. The negative deduction is can't have / couldn't have.

Mustn't have can be interpreted differently in some varieties and contexts. For a clear negative deduction, can't have or couldn't have is the safest standard choice.

Quick exercise

Complete each sentence with should have, must have, can't have, might have or had to, plus the verb in parentheses.

1. The office is dark and her coat is gone — she ___ (leave) already.
2. I ___ (book) earlier; there are no seats left now.
3. He ___ (read) my message — his phone was off all day.
4. I don't know why she's late. She ___ (miss) the bus, I suppose.
5. Sorry I missed dinner — I ___ (stay) at work until nine.
6. You ___ (tell) me it was her birthday! I'd have brought something.