MASTERING PERFECT MODALS 1

We can use modal verbs for deduction – guessing if something is true using the available information. The modal verb we choose shows how certain we are about the possibility. This page focuses on making deductions about the past.

We use must have + past participle when we feel sure about what happened.

FOR EXAMPLES:

The thief must have had a key. The door was locked and nothing was broken.

Who told the newspapers about the prime minister's plans? It must have been someone close to him.

We can use might have or may have + past participle when we think it's possible that something happened. 

FOR EXAMPLES:

I think I might have left the air conditioning on. Please can you check?

Police think the suspect may have left the country using a fake passport.

We use can't have and couldn't have + past participle when we think it's not possible that something happened.

FOR EXAMPLES:

She can't have driven there. Her car keys are still here.

I thought I saw Adnan this morning but it couldn't have been him – he's in Greece this week.

We use should have and would have + past participle to talk about imagined situation in the past.

We use should have to express advice the past (it implies the correct action was not done).

We use would have to suggest an alternative to the action taken

You should have sent a birthday card. (=You didn't send a card and that was a mistake.)

You shouldn't have been late. (=You were late and that was a mistake.)

Why did you go to France? I would have gone to Italy. (=In the same situation, I would have done something different.)