modal verbs - He mustn't / couldn't have been hungry

He couldn't have been hungry.

He mustn't have been hungry.

Is there a difference in meaning between those two?

Answer

(I think I'm going to open a can of worms with this answer but I've done some research so, don't blame me.)

In the student's text book, New English File Upper-Intermediate Oxford University press Page 138 it says:

The opposite of "must have" is "can't have" NOT "mustn't have"

So for some it is considered standard English to use: can't have or couldn't have instead of mustn't have when you are speculating or guessing about the past in questions and negative sentences.

• 
  

  He couldn't have been hungry

  
  
  

  means practically the same as

  
  


• 
  

  He can't have been hungry

  
  

They both express a strong conviction in the past, the speaker can choose to add further information in order to back up his claim.

A: John didn't eat his cereal this morning.

B: He can't/couldn't have been hungry. He usually has breakfast.

Thus the speaker is saying it's impossible that John was hungry because he knows John never leaves home without eating something. Must not (mustn't) means something quite different, you are forbidding someone or something from performing an action now, in the present and it is not used for speculating in the past.

On p394 in Practical English Usage by Michael Swan:

Must is used with the perfect infinitive for deductions about the past.

• "The lights have gone out" -- "A fuse must have blown."


• "We went to Majorca." -- "That must have been nice."

Must is only used in this way in affirmative sentences. In questions and negatives, we use can and can't instead.

This is also confirmed by A Practical English Grammar by A.J.Thomson A.V. Martinet 4th edition on page 148.