English as a Second Language/couldn't,,,,

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Question
Hi
I have two questions:

Question 1)
Which one is correct? And if both are correct, then what is the difference between them?

A) Paul didn't learn the lesson. He mustn't have listened to his teacher while his teacher was teaching.

B)  Paul didn't learn the lesson. He couldn't have listened to his teacher while his teacher was teaching.

Question 2: Which one is correct? And if both are correct, then what is the difference between them?

1) The police said it mustn't have been an accident.
2) The police said it couldn't have been an accident.

Thank you
Wish you all the best

Answer
Hi Hame,

Interesting questions here:

Question 1)

B) is correct: Paul didn't learn the lesson. He couldn't have listened to his teacher while his teacher was teaching.

Question 2

2) is correct: The police said it couldn't have been an accident.

As an American, I have to admit that mustn't have sounds like British English from the 19th century to me ;) The opposite of "must have" is "can't/couldn't have" NOT "mustn't have"

For ex: My mother must have opened the door for the dog to go out.
       My mother couldn't have opened the door for the dog to go out.

These two sentences have opposite meanings.

I hope this helps.

Best,

Amy  

English as a Second Language

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Amy Baker

Expertise

I can answer question about grammar, spelling, syntax, idioms, reading and/or writing that pertain to English as a Second Language. I am knowledgeable about both TOEFL and IELTS.

Experience

I'm a certified ESL teacher with 12 years of experience teaching K-12 and adults.

Education/Credentials
BSc MEd TESL post grad program for k-12 TESL post grad program for adult ed

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