The structure “have + past participle” is called a perfect infinitive.
Using Perfect Infinitives with Modal Verbs
MODAL + HAVE + PAST PARTICIPLE
- 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
Must + Have + Past Participle
Express deduction, a logical conclusion, probability
- It is very cold; it must have snowed in the mountains.
- If you can’t find your car keys, you must have left them in the house.
Can’t/Couldn’t + Have + Past Participle
Express negative deduction (impossibility in the past)
- She can’t have passed such a difficult exam.
Could + Have + Past Participle
Express past reference about something that was not carried out
- You could have done it. (You didn’t do it)
- She could have called the doctor early in the morning. (She didn’t call the doctor)
May + Have + Past Participle
Express 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.)
Might + Have + Past Participle
Express a past possibility
- Our neighbours might have heard some noises when our car was stolen.
Needn’t + Have + Past Participle
Express 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.
Should + Have + Past Participle
Indicate 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.)
Ought to + Have + Past Participle
Express 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.)
Would + Have + Past Participle
The third conditional
- I would have gone to university if my parents had had more money. (The speaker didn’t go to university.)
Perfect Infinitives with Modal Verbs | Image