We don't use 'must have' or 'can't have' in question form because we use them for definite beliefs. We can use 'yet' with negative forms e.g. We can use 'by now' with positive forms e.g. 'might not' can be contracted to 'mightn't'. We can make a negative form of 'might have' with 'not': we believe it is possible that the action did not happen, but it is also possible that it did. We use 'can't have' in the negative form only: we believe that the action definitely did not happen (the opposite of 'must have'). We use 'must have' in the positive form only: we believe that the action definitely happened. subject + 'might (not) have' + past participle likelihood of the deduction.subject + 'can't have' + past participle.subject + 'must have' + past participle.We use 'must have', 'can't have' and 'might have' with the past participle of the main verb: The real situation and result: I didn’t know about the traffic problem so I am imagining the possibility of taking a different route if I had known. The clause with might have describes the possible result of the unreal situation described by the 'if' clause.'If I had known about the traffic problems, I might have taken a different route.'.'might have' / 'might not have' in the third conditional structure: to imagine the possible result of an unreal situation in the past.It is possible that he has not finished his exams, but it is also possible that he has finished them.'He might not have finished his exams yet.'.'might not have': we believe it's possible that the action did not happen, but we don't know.It is possible that she has gone to the shops, but it is also possible that she has gone somewhere else.'might have': we believe it's possible that the action happened, but we don't know.She usually takes her car so I deduce that she has definitely not left the house because her car is outside (the opposite of 'must have').'She can’t have left the house yet because her car is still outside.'.'can't have': we believe the action definitely did not happen (the opposite of 'must have').She usually leaves her house before 11 o’clock so I deduce that she has definitely left the house.'She must have left the house by now it’s nearly 11 o'clock.'.'must have': we believe the action definitely happened.We use 'must have', 'can't have' and 'might have' in the same way as the present perfect - the action we are describing happened, or did not happen, in the past and is still true in the present. We use the modal verbs 'must have', 'can't have' and 'might have' to make guesses or deductions about an action in the past that we believe has definitely happened, has definitely not happened or possibly happened, based on our knowledge, information or evidence, or lack of it.

Modal verbs in the past exercises