The form modal + have + past participle expresses the speaker's present conjecture, evaluation, or regret concerning a past event. It is one of the most important patterns at the high-school level.
Basic Rules
Uses of modal + have + past participle:
① must have + past participle: “must have done” (certainty about the past) “He must have been very tired.（He was certainly very tired）”
② may/might have + past participle: “may have done” (a cautious conjecture about the past) “She may have missed the train.”
③ cannot have + past participle: “cannot have done” (strong negative conjecture about the past) “He cannot have said such a thing.（There is no way he said such a thing）”
④ should have + past participle: “should have done, but actually did not” (regret or criticism about the past) “You should have told me.（You should have told me）”
⑤ shouldn't have + past participle: “should not have done” “I shouldn't have eaten so much.”
⑥ need not have + past participle: “it was unnecessary to do something, but it was actually done” “You need not have come so early.”
⑦ could have + past participle: “could have done, but actually did not” or “may have done” “I could have helped you.（I could have helped you, but I did not）”
⑧ would have + past participle: past perfect subjunctive “If ~, I would have done” (explained in detail in Lesson 8)
She must have forgotten the appointment.（She certainly forgot the appointment）
He may have left already.（It is possible that he has already left）
You should have studied harder for the exam.（You should have studied harder for the exam）
She cannot have done such a terrible thing.（There is no way she did such a terrible thing）
I could have called you, but I forgot.（I was able to call you, but I forgot）
Points to Remember
① Degrees of certainty in conjecture:
- must have done (90%): must have done
- may have done (50%): may have done
- might have done (30%): may have done (more tentative)
- cannot have done (90% negative): cannot have done
- could have done: could have done / may have done (two meanings)
② should have vs ought to have: Both express nearly the same regret, “should have done.” ought to have done is also frequently tested.
③ Difference between need not have done and didn't need to do:
- need not have done: it was unnecessary, but it was actually done “I need not have brought my umbrella. (I brought my umbrella, but it was unnecessary)”
- didn't need to do: it was unnecessary, so it was not done “I didn't need to bring my umbrella. (It was unnecessary to bring my umbrella, so I did not bring it)”
This is a frequently tested trick in university entrance examinations.
④ The two meanings of would have done: the subjunctive (if ~) and simple conjecture about the past. “He would have been a great writer if he had lived longer.(subjunctive)” “Who's that? — That would have been Tom.(conjecture, formal)”.
- must have done: must have done (certainty)
- may/might have done: may have done (conjecture)
- cannot have done: cannot have done (negative certainty)
- should have done: should have done (regret or criticism)
- need not have done (done) vs didn't need to do (not done)
Practice
- Translate “She must have gotten lost” into English using must have.
- Translate “You should have left home earlier” into English using should have.
- Explain the difference between the two sentences in English: (a) I need not have hurried. (b) I didn't need to hurry.