A: I'm sure he stole the money.
B1: No. He couldn't have done that!
B2: No. He wouldn't have done that!
B2: No. He mightn't have done that!
May I ask the difference between B1,B2 and B3?
Grammar – How to Understand ‘He Wouldn’t Have Done That!’
grammar
Best Answer
B1 is saying that he literally could not have done it, which implies that the speaker believes that it was impossible for the man to have done it.
B2 is saying that he would not have done that, which implies that the speaker doesn't believe that the man did it. This shows that speaker is familiar with the man, and knows him well enough to know that what was done was not something the man would voluntarily do.
B3 is saying that he might not have done that, which means that the speaker doesn't feel too strongly about whatever happened. The speaker is pointing out that there is the possibility that the man did not do it, but in doing that, the speaker is also pointing out the possibility that the man did do it.