Is there any difference between "mustn't have+p.p " and couln't have +p.p ?

1.martha couldn't have taken your notebook,she wasn't even in class.

2.martha mustn't have taken your notebook,she wasn't even in class.

Also ,is there any differences between "mustn't" and "can't" ?

1.He looks lazy. He mustn't be a policeman or That mustn't be right answer.

2.He looks lazy. He can't be a policeman or That can't be right answer.

Again, I also want to know: can i use "couldn't" in the same way in present tense in the place of 'can't' ? For example:

*It couldn't be the right answer.

So,please make me clear about mustn't have+p.p/couldn't have+p.p/mustn't/can'tand couldn't ?


could is about possibility- it is the past of can, and it's about being able to do something. can't and couldn't means that something is impossible.

must is about probability- you use it when you confidently infer something from information that you have.

  1. Martha couldn't have taken your notebook, she wasn't even in class.

This means that it is impossible that Martha could take your notebook. She was not able to, because she was not in the class.

In this sentence, you would use couldn't because it really was impossible that Martha took the book. It's not a matter of probablility, it's a fact. I will therefore use a different example:

2a. Martha missed the class: she must'nt have heard that the time had changed.

2b. Martha missed the class: she must have not heard that the time had changed.

We infer that Martha probably did not know that the class time had changed. The information that we have is that we know that she missed the class. Most people would choose the second form, with not after have.

Neither of the Policeman examples work, so I will move on to the "right answer" examples.

  1. That mustn't be right answer

Imagine that you are doing a crossword puzzle. You have filled in an answer for 1 across, then you read the clue for 1 down and you are sure that you know the right answer for it, but the first letter isn't the same as the first letter of the answer for 1 across. So you say example 3, which indicates that you infer that the answer to 1 across is probably wrong. The information you use is that you are sure that the answer to 1 down is right, and that doesn't fit with this answer for 1 across.

  1. That can't be the right answer

Imagine that you are solving a math puzzle and the question says that the answer must be an even number. You do the sums and get an answer of 13. You know that the answer must be an even number, so is is impossible for 13 to be the right answer.

  • 1
    "Most people would choose the second form, with not after have." - might be a regional thing, but I would expect to hear 2a, not 2b. "Mustn't have heard" or without the contraction "must not have heard" both sound much more natural to me than "must have not heard". – nnnnnn May 25 '16 at 12:57
  • @nnnnnn... possibly. – JavaLatte May 25 '16 at 13:00
  • because i think: There is alwas a complexcity in a specific context of a language to understand the exact meaning of the sentence that is talked in a way which tends to be percieved differently by a non native speaker or they think differently comparing to their own language, that's what makes us difficult to learn a language. But, Natives are unware of this issue – yubraj May 25 '16 at 13:27
  • Javalatte@ please remove the example from math and add other kind of examples because I'm not understanding it. I also want to request not to use complex word in your examples. Please do it for me. I will upvote or accept your answer your answer after that. – yubraj May 25 '16 at 13:33
  • I'm not understanding the example of math – yubraj May 25 '16 at 13:34

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