Study materials
These two sentences are about the same umbrella, but they say opposite things.
You should have taken an umbrella.
You didn't and got wet.
You needn't have taken an umbrella.
You took it, and it didn't rain.
Past modal forms do more than guess what happened. They also judge whether something was worth doing. The article explains should have, could have as a reproach, could have or was able to, needn't have, and the difference between must have and had to.
How the form is built
Should, could, needn't and ought move into the past the same way as must: the modal, have and the third form. Have does not become has, and to comes only after ought: should have called, could have told, needn't have bought, ought to have booked. The topic Must Have, Can't Have, Might Have explains the form in detail.
Should have
Should have and shouldn't have judge an action. They criticise others or express regret about yourself. Should have says the action was worth doing and was not done. Shouldn't have says it was done and was not a good idea.
You should have told me.
You didn't.
I shouldn't have eaten that third burger.
I did.
Ought to have means the same as should have but sounds more formal and is less common. The to is required: We ought to have booked earlier.
Should have has one more meaning, expectation. The parcel should have arrived by now means that, according to plan, it was due by now. The speaker does not know whether it has arrived. So should have does not always mean it did not happen.
Could have as a reproach
Could have means something was possible but did not happen. Said to a person, it sounds like a reproach: you could have done it, but you did not. Might have is also used this way.
You could have called! I was worried.
He might have said thank you.
The meaning depends on the situation. She could have missed the train is a guess. You could have told me is a reproach, because the speaker knows nobody told him.
Needn't have and didn't need to
Needn't have with the third form means the action was done but was not necessary. The effort was wasted.
You needn't have brought a cake. We already had two.
The cake was brought.
I needn't have hurried. The train was late.
I did hurry.
Didn't need to with the base form only says there was no need. It is used most often when the action was not done. But whether it was done shows only from the context.
I didn't need to take a taxi, so I walked.
Needn't have differs from shouldn't have. Needn't have says the action was unnecessary. Shouldn't have says it was a bad idea.
Could have or was able to
Could have with the third form talks about a chance that was not taken. Was able to, managed to or simply a verb in the Past Simple say that the action succeeded.
Could without have describes a general ability in the past: She could swim at five. It is not used for one particular success; was able to or managed to is needed then.
I could have taken the earlier flight.
But I didn't.
I was able to take the earlier flight.
And I did.
We managed to find a parking space.
So could have sounds wrong in a story about a real success. It says there was a chance but no result. For a single failure couldn't works: I couldn't find my keys.
Could have been with the third form often describes a danger that was avoided: You could have been killed! Nobody died, but it was possible.
Must have, had to and should have
These three forms are easy to mix up, because in Ukrainian they can all sound like «мав». Must have draws a conclusion: He must have overslept. Had to talks about an obligation that was carried out: He had to get up at five. Should have judges: He should have set an alarm.
Common mistakes
The first mistake is to after should.
You should to have told me. → You should have told me.
To comes only after ought.
The second mistake is must have for an obligation.
My car broke down, so I must have taken a bus. → My car broke down, so I had to take a bus.
Past obligation is had to.
The third mistake is shouldn't have where the action was just unnecessary.
Thanks for the flowers, but you shouldn't have bought so many. → Thanks for the flowers, but you needn't have bought so many.
Buying flowers was not bad, just more than needed.
The first mistake shows right after should. Have stands there, and to appears only after ought. The second shows if you ask whether it is a conclusion or an obligation. If you were obliged to do something, had to is needed. The third shows if you ask whether the action was bad or just unnecessary. An unnecessary action is described with needn't have. Still, as a polite reply to a gift, you shouldn't have sounds natural too.