Third conditional sentences are used to explain that present circumstances would be different if something different had happened in the past. Look at the following examples:

If you had told me you needed a ride, I would have left earlier

If I had cleaned the house, I could have gone to the movies.

These sentences express a condition that was likely enough, but did not actually happen in the past. The speaker in the first sentence was capable of leaving early, but did not. Along these same lines, the speaker in the second sentence was capable of cleaning the house, but did not. These are all conditions that were likely, but regrettably did not happen.

Note that when using the third conditional, we use the past perfect (i.e., had + past participle) in the if-clause. The modal auxiliary (would, could, shoud, etc.) + have + past participle in the main clause expresses the theoretical situation that could have happened.

Consider these common mistakes when applying the third conditional:

Incorrect: If you would have told me you needed a ride, I would have left earlier.

Correct: If you had told me you needed a ride, I would have left earlier.

Explanation: With third conditional sentences, do not use a modal auxiliary verb in the if-clause.

Incorrect: If I had cleaned the house, I could go to the movies.

If I had cleaned the house, I could have gone to the movies.

Explanation: The third conditional mood expresses a situation that could have only happened in the past if a certain condition had been met. That’s why we use the modal auxiliary verb + have + the past participle.

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note about the conditionals in English Conditional 0 Conditional 1 Conditional 2 Conditional number 3