It is possible that we won't have to take an entrance exam this year.

•  Perhaps we don't have to take an entrance exam this year.

• We mustn't take an entrance exam this year.

• We mightn't take an entrance exam this year.

• It is likely that we will take an entrance exam this year.

Perhaps he had returned before you called.

• He may be returning before you called.

• He may/might have returned before you called.

• He should have returned before you called.

• He must have returned before you called

 It would have been better if she had bought some sugar.

•  She should have bought some sugar

• She must have bought some sugar.

• She might buy some sugar. 

• She might have bought some sugar.

It turned out that I had been worrying about my job unnecessarily. It was not as difficult as I thought.

• I shouldn’t have worried about my job.

• My job turned out to be worried.

•  I turned out to be worrying about my job.

•  I needn’t have worried about my job.

I’m sure he knew all about the secret because he was there when we were talking

• He must be knowing all about the secret because he was there when we were talking.

• He should have known all about the secret because he was there when we were talking

• He must have known all about the secret because he was there when we were talking.

• He might have known all about the secret because he was there when we were talking.

It isn't mandatory to submit your assignment today.

• You mustn't submit my assignment today.

• My assignment must have been submitted today.

• I needn't submit my assignment today.

• My assignment is required to submit by today.

Tom went on and on apologizing about it, which was quite unnecessary.

• Tom can't have apologized. I quite understand.

• Tom shouldn't have apologized. I quite understand.

• Tom wouldn't have apologized. I quite understand.

• Tom needn't have apologized. I quite understand.

It’s possible that she didn't hear what I said.

• She might have not heard what I said.

• She might not hear what I said.

• She may not hear what I said

• She may not have heard what I said.

It was a mistake for her to marry Peter.

• She shouldn't have married Peter.

• She and Peter weren't married in the right way.

• Peter didn't want to get married, so it was his mistake.

• She ought to think again before she marries Peter.

It was wrong of you to betray her because she was kind to you.

• You shouldn’t have betrayed her because she was kind to you.

• You needn’t have betrayed her because she was kind to you.

• You can’t have betrayed her because she was kind to you.

• You mustn’t have betrayed her because she was kind to you.