To refer to the PAST. To refer to Unreal Situations. To show that the activity was different from

what we wanted. To say how confident we are that something

has happened.

Subject + modal + have + Past participle

•I•We•They•The students•My son

shouldcanmightought to

goneplayedvisitedwonpaid

have

It expresses deduction, a logical conclusion, probability:

It’s very cold; it must have snowed in the mountains. I saw him leaving. He must have finished the test.

It expresses past reference about something that was not carried out.

She could have called the doctor early in the morning. They could have danced in the party.

It expresses negative deduction, impossibility or disbelief:

She can’t have passed the exam. It was too difficult. The boy couldn’t have given a better answer than this

one. Bryan can’t have missed the bus.

It expresses the possibility that an action took place in the past (past possibility). The little boy may have lost the key. The little boy might have lost the key.

Our neighbours may have heard some noises when our car was stolen.

Our neighbours might have heard some noises when our car was stolen.

With “may”, action is more probable than with might.

The action was not performed because of external reasons. I would have played football but I had a strong pain in my leg.

Past Unreal Conditional. I would have gone to Paris if I had had the money for the

plane ticket.

It expresses an unnecessary action, which was performed. I needn’t have knocked at the door in this way. I awoke

the baby. You needn’t have bought the flowers for me.

In contrast, “didn’t need to” shows that the action was not necessary and it wasn’t performed.

I didn’t need to knock at the door because it was open (so I didn’t knock)

It expresses that a past duty or obligation was not carried out.

You should have locked the door before leaving the house (but you didn’t lock it).

Paul ought to have waited until the lights were green before he crossed the street (but he didn’t wait).