A. How to use modal verbs to speculate about past events?
MUST
CAN'T
MAY/MIGHT
+ bare perfect infinitve
We believe that the action definitely happened.
» He must have left his job.
» She must have studied all night, she looks tired.
+ bare perfect infinitive
We believe the action definitely did not happen, the opposite of must.
» She can’t have married him!
» He can’t have done it!
+ bare perfect infinitive
Something probably happened in the past, or possibility which did not happen.
» He may have come back.
» She might have changed her mind.
» You might have cut your finger!
COULD
SHOULD/
OUGHT TO
NEEDN'T
+ bare perfect infinitive
To say that something was possible to do in the past, or you had the ability to do something in the past, but you didn’t do it.
» You could have helped her. (so why didn’t you?)
» Peter is late. What could have happened to him?
» If I married John, I could have been a millionaire. (potential possibility)
+ bare perfect infinitive
To express criticism or regret about a past event, or to describe a situation you expect to happen.
» You ought to have called sooner.
» You shouldn’t have gone there.
+ bare perfect infinitive
It wasn’t necessary to do it, but it was done.
» You needn’t have bought the ham, I have bought it. (but he bought it)
» You needn’t have done so much trouble. (but he did)
Note:
DIDN’T HAVE TO/ DIDN’T NEED TO
+ present infinitive
It wasn’t necessary to do it and it wasn’t done.
» I didn’t have to get up early, it was Sunday.
» He managed to do it himself, I didn’t have to help him.
B. How to form it?
Modal verb + bare perfect infinitive
-
- Perfect bare infinitive: She may have written a letter.
- Perfect continuous bare infinitive: She may have been writing a letter.
- Perfect bare infinitive passive voice: The letter may have been written by her.