could have - must have - may have - might have

Question

Why is John late?

He could have got stuck in traffic.
He must have been stuck.
He may have been stuck.
He might have been stuck.

Are there any differences (even subtle) between these answers ?

Answer 1

He could have got stuck in traffic.

An action he took in the past resulted in the possibility of him getting stuck in traffic in the past.

Since this possibility has passed, it ended up that he did not get stuck in traffic. Otherwise we would say "He did get stuck in traffic."

E.g. Why is Bobby late? Forgive him, he could have been stuck in the rain.

It also can be used in the same way as "might have been" below.

He must have been stuck.

Something happened X, and the only reason that explains it is the fact that he had been stuck. X refers to something that is completed or completely in the past. E.g. Bobby was late because he must have been stuck in the rain. (Bobby is here now)

He may have been stuck.

Something happened X, and a possible reason that explains it is the fact that he had been stuck. Other reasons for him being stuck are possible. X refers to something that may have started in the past but is happening now. E.g. Bobby is not here, he may have been stuck in the rain. (We are still hoping he makes it)

He might have been stuck.

Something happened, and a possible reason that explains it is the fact that he had been stuck. Other reasons for him being stuck are possible. X refers to something that is completed or completely in the past. E.g. Bobby hasn't called. He might have been stuck in the rain. (Implication is that we don't expect him to show up)

Answer 2

Could have (been) done emphase you had the ability to achieve some thing if you had tried some other things.

for example, you could have passed the exam if you studied hard.

May, Might and Must describle the magnitude on the possiblity that some thing is happening or just happened.

the Must has the strongest estimation, it is almost certain. May has the medium estimation whereas Might has the least estimation.

Answer 3

In each of these, the speaker is making an inference or guess. But there are differences in certainty.

When must have is used, the speaker is presuming it to be true. They aren't completely certain, but they will assume it.

With the other 3 phrases, they are making no assumptions, but instead are stating that it is a possibility that John was stuck in traffic. Personally, I don't see much difference in their meanings in this case, though some might claim that one is less certain than another.

However, a note should be made about could have. Although in these examples it closely resembles may have and might have, there are other uses of could have where the others can't be substituted:

I heard John took all day to fix that bug. I could have fixed it in 5 minutes.

In this case, could have is used to state that something was a real possibility in the past, even though it didn't really happen. In the above example, the speaker didn't fix the bug, but if they had tried, they would have.

Answer 4

Could, may and might indicate less certainty than must. Must is almost completely sure.

Answer 5

He could have got stuck in traffic.

It is about possibility

He must have been stuck.

It is about certainty

He may have been stuck.
He might have been stuck.

Both are equivalent and both are about probability