Perfect modals: you might have told me

You might have told me, she'll have left by now, I could have cried, I'd rather you hadn't.

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Modal + have + past participle looks back at the past. At B2 you used it for deduction (must have, can't have), for criticism and regret (should have) and for things that were not necessary (needn't have). At C2 the same forms carry finer shades: a reproach, a confident assumption, a past that nearly happened, or a strong feeling. The context, and often the stress, decides which.

Form	Shade of meaning	Example
might have / could have	a reproach: you didn't, and you should have	You might have told me!
will have / would have	a confident assumption about the past	She'll have left by now.
might have / could have	it nearly happened, but didn't	You could have been killed.
could have	a strong feeling	I could have cried.
needn't have	done, but not necessary	I needn't have worried.
would rather + subject + past perfect	a wish about someone's past action	I'd rather you hadn't told her.

Reproach: you might have told me

Might have and could have + past participle can criticise someone for not doing something. The modal is usually stressed. Might have in this sense is more common in British English and can sound annoyed or ironic.

You might have told me you were bringing friends!
You didn't tell me, and you should have.
You could have asked before borrowing my car.
He might at least have apologised.
At least makes the reproach stronger.

Should have and ought to have say the same thing more directly: You should have told me.

Assumptions: she'll have left by now

Will have + past participle says what you are confident has happened, from what you know. Would have makes a similar guess about the past, often when you say who someone was. Should have says what you expect has happened, but you are less sure.

It's past six. She'll have left the office by now.
"Someone called while you were out." "That would have been my brother."
They left at nine, so they should have arrived by now.
I expect so, but I'm not certain.
Incorrect: It's past six, so she will leave the office by now.
Correct: It's past six, so she will have left the office by now.
By now looks back at a finished action, so use will have + past participle.

It nearly happened: you could have been killed

Could have and might have + past participle can describe something that was possible but did not happen. May have usually means that we don't know what happened. Many speakers use may have for both, but careful writers keep might or could for what did not happen.

You could have been killed! Why didn't you wear a helmet?
You weren't killed, but it was possible.
The fire might have spread to the whole street if the wind had changed.
He may have missed the train. I'll check the timetable.
We don't know yet whether he missed it.

Strong feelings: I could have cried

Could have + a verb such as cried, screamed, killed or hugged is a common exaggeration: the feeling was so strong that you almost did it. Couldn't have + comparative means "it was the best or worst possible".

When I saw the bill, I could have cried.
I could have killed him for telling her!
I was very angry.
The interview couldn't have gone better.
It went perfectly.

Needn't have, didn't need to and shouldn't have

Needn't have + past participle means you did something that was not necessary. Didn't need to usually means it wasn't necessary, so it wasn't done. You shouldn't have! is a polite thank you for a present: it was kind, but it wasn't necessary.

I needn't have worried. Everyone was very friendly.
We didn't need to book. The restaurant was half empty.
Most likely we didn't book.
"This is for you." "Oh, you shouldn't have! It's lovely."

Would rather + subject + past perfect

Would rather + another person + past perfect says you wish they had acted differently in the past. Would rather have + past participle is about your own past choice.

I'd rather you hadn't told her about the party.
You told her, and I wish you hadn't.
I'd rather have stayed at home. The party was awful.
I went, and I wish I hadn't.
Incorrect: I'd rather you didn't tell her about the party yesterday.
Correct: I'd rather you hadn't told her about the party yesterday.
For a past action, use the past perfect after would rather + subject.