Modal verbs (past deductions)

When we know that something happened in the past, we use normal past tense. (He did it, I saw them, etc.). If we are not completely sure, we use a modal (must, may, might, would, could + have + Past Participle). The modal verb we choose shows how certain we are about the possibility. This page focuses on making deductions about the past. 

PROBABILITY

ALMOST CERTAIN, QUITE LIKELY,  50% (POSSIBLE), POSSIBLE BUT UNLIKELY, HIGHLY UNLIKELY 

S + MUST/MAY/COULD/MIGHT/CAN’T/COULDN’T + HAVE + PAST PARTICIPLE

Use MODAL PERFECTS to make guesses (deductions) about things in the past. The MODAL PERFECTS is formed with a modal verb + must, can, should + have + past participle. 

MUST HAVE + PAST PARTICIPLE

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

You must have been tired. You didn’t respond to my messages last night. 

He must have gone home. 

MIGHT HAVE + PAST PARTICIPLE

We use might have + past participle to make an assumption/guess about the past. We use this if we want to say that something probably happened in the past.  

I might have lost something. I think I was running at that time. 

He might have jumped the fence to enter his aunt’s house. 

COULD HAVE + PAST PARTICIPLE

We use could have + past participle to make an assumption/guess about the past. Could have and might have can be interchangeable and mean the same thing. 

I could have lost something. I think I was running at that time. 

He could have jumped the fence to enter his aunt’s house. 

We also use could have + past participle when talking about something that could have happened, but did not. 

You could have broken your leg! Be careful next time. 

I could have gone to the cinema, but I decided to stay home. 

Whereas negative form couldn’t have + past participle is used to talk about something that couldn’t have happened and something impossible. 

She couldn’t have met him before. He used to live in Canada. 

He couldn’t have gone to the shopping center. It’s closed on Sundays. 

SHOULD/OUGHT TO HAVE + PAST PARTICIPLE

We use should/ought to have + past participle when we mean something that would have been a good idea but that you didn’t do it. It’s like giving advice about the past when you say it to someone else, or regretting what you did or didn’t do when you are talking about yourself. 

We use should/ought to have + past participle when giving advice or suggesting what seemed to be the best solution in a given situation in the past (but probably did not happen. 

You should have taken that medicine. 

You ought to have spent more time with your family. 

We use should/ought to have + past participle when talking about past mistakes. 

I was so worried about you. You should have called me.

She got sick last night. You should have given her some medicine. 

We use should/ought to have + past participle when giving apologies. 

I am sorry that I’m late for work. I should have woken up earlier. 

I’m so sorry for the delay in the project. I ought to have submitted the project proposal a bit earlier. 

WOULD HAVE + PAST PARTICIPLE

Would have + past participle is a bit more difficult because it has two common structures. The first is with the conjunction ‘but’.

To talk about something you wanted to do but didn’t

I would have gone to the meeting, but I was really busy. 

I would have lent you the money, but I didn’t have any. 

Part of the third conditional

If I had known that she was hospitalized, I would have visited her. 

If I had studied hard, I would have passed the test.