Introduction

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.

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.

• "must have": we believe the action definitely happened.
  • "She must have left the house by now; it’s nearly 11 o'clock."
  • She usually leaves her house before 11 o’clock so I deduce that she has definitely left the house.
  
  
• "can't have": we believe the action definitely did not happen (the opposite of "must have").
  • "She can’t have left the house yet because her car is still outside."
  • 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").
  
  
• "might have": we believe it's possible that the action happened, but we don't know.
  • "She might have gone to the shops."
  • It is possible that she has gone to the shops, but it is also possible that she has gone somewhere else.
  
  
• "might not have": we believe it's possible that the action did not happen, but we don't know.
  • "He might not have finished his exams yet."
  • It is possible that he has not finished his exams, but it is also possible that he has finished them.
  
  
• "might have" / "might not have" in the third conditional structure: to imagine the possible result of an unreal situation in the past.
  • "If I had known about the traffic problems, I might have taken a different route."
  • The clause with might have describes the possible result of the unreal situation described by the "if" clause. 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.
  
  

Form

We use "must have", "can't have" and "might have" with the past participle of the main verb:

• subject + "must have" + past participle
• subject + "can't have" + past participle
• subject + "might (not) have" + past participle

likelihood of the deduction	Subject	modal	main verb (past participle)
definite	I You We They He / She / It	must have	been seen taken spoken	to France. the movie. the medicine. to the manager.
definitely not		can't have
possible		might (not) have

Positive and negative forms

We use "must have" in the positive form only: we believe that the action definitely happened.

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 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. "might not" can be contracted to "mightn't".

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Using "by now" and "yet"

We can use "by now" with positive forms e.g.

• "He must have done it by now."
• "She might have done it by now."

We can use "yet" with negative forms e.g.

• "He can't have done it yet."
• "He might not have done it yet."

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Question form

We don't use "must have" or "can't have" in question form because we use them for definite beliefs. We use "might have" for question forms:

Positive question form

modal	Subject	"have"	main verb (past participle)
Might	I you we they he / she / it	have	been	to France?

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Practice

Complete the sentences with "can't have", "must have" or "might (not) have" using the verb in brackets:

• He hard for his exams because his results were very good. (study)
   
• They the instructions because they looked confused. (not understand)
   
• She home late last night because she was very tired this morning. (get)
   
• they to the wrong place? (go)
   

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Further practice

Complete the sentences with the correct answer from a, b, c or d:

• Sarah thinks she ____ her purse on the bus.


must have left must has left can has left might has left



• She ____ the train instead of the bus.


must have took must has taken might have taken can't have took



• They ____ the project yet because they only started work on it this week.


can't have finished might haven't finished might not have finish must have finished



• She ____ received the invitiation yet.


can't not has might not has must not has can't have



• If he had understood the consequences, he ____ the email.


must not have written might not have written might not have write can't have written



• He is late for this appointment. His train ____ delayed.


can has been must has been might has been might have been

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Further language point - "might have" with "not"

"might have not"

In addition to the most common form "might not have" (described above), it is also possible to use the form "might have not":

• "He might not have invited me to the party."
• "But he might have not sent out the invitations yet."

In the second sentence, "not" is used to emphasise the negative of the main verb; in this case, "might have" describes a positive possibility that he did not send the invitations.

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Negative question form

We can also use "might not have" to form questions. We can change the position of "not" to make different emphases:

modal	Subject	"have"	main verb (past participle)
Might	I you we they he / she / it	not have	been	to France?
		have not
Mightn't		have

• "Might they not have been to France?"
• "Mightn't they have been to France?"
  In these examples, we are emphasising the possibility that they have been to France, for example in response to a suggestion or assumption that they have not.

• "Might they have not been to France?"
  Here, we are emphasising the possibility that they have not been to France, for example in response to a suggestion or assumption that they have.