2.  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.

3. Subject + modal + have + Past participle I We They The students My son should can might ought to gone played visited won paid have

4.  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.

5.  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.

6.  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.

7.  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.

8.  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.

9.  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)

10.  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).