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EyeSeeYou Posted 13 years ago
Grammar

Inference: can't have/couldn't have; mustn't have

When talking about deduction in the past, are can't have + past particple, and couldn't have + past participle interchangeable? Or is there a slight difference between them?

Joe can't have arrived at the party yet; he has a one-hour drive at least.

Joe couldn't have arrived at the party yet; he has a one-hour drive at least.

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As for MUSTN'T HAVE + past participle. Does it even exist? I know the opposite of can't have been is must have been, for instance.
  

Top answer

EyeSeeYou As for MUSTN'T HAVE + past participle. Does it even exist? I know the opposite of can't have been is must have been, for instance.

— AlpheccaStars

  • EyeSeeYou As for MUSTN'T HAVE + past participle.
  • Does it even exist?
  • I know the opposite of can't have been is must have been, for instance.
  • It is used in American English.
  • I use it when the situation calls for it.
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9 Answers
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EyeSeeYouAs for MUSTN'T HAVE + past participle. Does it even exist? I know the opposite of can't have been is must have been, for instance.
It is used in American English. I use it when the situation calls for it. (eg. He failed the exam. He mustn't have studied hard enough.)
It is my impression that the British English speakers don't use it.
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EyeSeeYouJoe can't have arrived at the party yet; he has a one-hour drive at least.Joe couldn't have arrived at the party yet; he has a one-hour drive at least.
To my mind, "can't" carries a greater degree of certainty and is also more emphatic. They both, however, indicate disbelief.
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Thanks both of you.

I remember reading somewhere once that "mustnt't have + past paticiple" could mean lack of necessity in the past but it just doesn't sound right in my mind.
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EyeSeeYouI remember reading somewhere once that "mustn't have + past participle" could mean lack of necessity in the past
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I think,  to  indicate lack of necessity in the past,  we  could use  "didn't have to".

I didn't have to accompany her. (However, I could have if I wanted.It wasn't necessary, It was my choice)

Am I right, AlpheccaStars and CJ?
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LaboriousI didn't have to accompany her. (However, I could have if I wanted.It wasn't necessary, It was my choice)
The auxiliary "have to" is used as much as "must" for obligation:

Mother: Son, you have to get up. You have to go to school today.
Son: Aw, Mom, do I HAVE to? (Emphasis on "have")
Mother: Yes, you do. And you hav
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AlpheccaStarsThe auxiliary "have to" is used as much as "must" for obligation:
Thank you AlpEmotion: stars,
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Laboriousto indicate lack of necessity in the past, we could use "didn't have to"
Yes.

must not doesn't work for this meaning, by the way.

You didn't have to get me a present for my birthday.
is not the same as
You mustn't get me a present for my birthday.
nor the same as
You mustn't have gotten me
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LaboriousMust implies an internal obligation, whereas "have to" implies an external obligation. Something that is a rule or law.Here's an example:I must lose weight. (It's my decision. No one has told me or forced me to do so)I have to lose weight. (Someone has told me to lose weight, probably the doctor)
Sorry for butting in, but I want to go on record as som
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