✅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.
❌ 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.