MODAL VERBS + PERFECT INFINITIVE (PI) Form: MODAL + HAVE + PAST PARTICIPLE - modal verbs + PI are often used:
- 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
1. MUST + PI: expresses deduction, a logical conclusion, probability:
It is very cold; it must have snowed in the mountains.
I saw him leaving; he must have killed him.
2. CAN´T/COULDN´T + PI: expresses negative deduction:
She can´t have passed such a difficult exam.
CAN + PI: expresses impossibility or disbelief:
The boy can´t have given a better answer than this one.
She can´t have missed the bus.
3. COULD + PI:
You could have done it. (past reference: didn´t do it)
You could do it. (future reference: possibility)
4. MAY + PI: expresses the possibility that an action took place in the past:
The little girl may have lost the key. (It is possible that she lost the key.)
(with may – action is more probable than with might)
5. MIGHT + PI: expresses a past possibility
Our neighbours might have heard some noises when our car was stolen.
MIGHT + PI continuous: expresses a possible action which was continuing at a
certain moment in the past: The kids might have been watching a cartoon at that time.
6. NEEDN´T + PI: expresses an unnecessary action, which was, nevertheless,
performed: I needn´t have knocked at the door since, in this way, I awoke the baby. (but I
knocked)
You needn´t have bought the flowers.
didn´t need to: show that the action was not necessary but it wasn´t performed either:
I didn´t need to knock at the door since it was open. (so I didn´t knock)
7. SHOULD + PI: indicates that the past obligation was not fulfilled or carried out:
You should have locked the door before leaving the house. (But you didn´t lock it.)
8. OUGHT TO + PI: expresses an unfulfilled duty or obligation:
Paul ought to have waited until the lights were green before he crossed the street. (But he
didn´t wait.)
9. WOULD + PI: 3rd conditional I would have gone to university if my parents had had
more money. (The speaker didn´t go to university.)