Past Modal

Past Modal 

- Use must / may / might / couldn't / can't / should + have + past participle to make deductions or speculate about past actions.

- must 

Use must have when we are almost sure that something happened or was true.

eg. He must have bought a new car.

- might / may have

Use might / may have when we think it's possible that something happened or was true. We can also use could have with this meaning.

eg. You might have left your bag on your desk in the classroom.

- couldn't have

Use coul'd have when we are almost sure something didn't happened or that it is possible. We can also use can't have.

eg. They couldn't have seen us. It was too dark.

- should have

Use should have to say that so mebody didn't do the right thing. We can use ought to as an altrnative to should have. However, should have is more common, especialy in speaking.

eg. You should have learned some Korean before you went.