Third Conditional If-Clauses in English Grammar

What is the third conditional?

The third conditional, also known as type-III if-clause, looks back at a past situation and its outcome and imagine them as different. It is the only conditional structure that talks about the past rather than the present or future.

I didn’t study. I didn’t pass the exam. (reality)
but: If I had studied, I would have passed the exam. (imagined past)

Because third conditional if-clauses talk about a past situation that did not happen, they are also known as the impossible conditional.

When to use third conditional if-clauses

Typical contexts for third conditional if-clauses include:

  • expressing regrets
If I had known about the dessert, I wouldn’t have eaten so much beforehand!
  • giving criticism and feedback
Your party would have been better if you had invited more people.
  • reflecting on the past
It’s crazy, if I had left the party five minutes earlier, we never would have met!

How to form the third conditional

The third conditional is perhaps the trickiest conditional structure. The if-clause contains the past perfect, while the main clause contains would have + past participle.

If we had left sooner, we would have seen the sunset.

One or both clauses can be negative. The main clause can contain a question.

We wouldn’t have told everyone if we had known it was a secret.
You wouldn’t have had a problem if you hadn‘t waited so long.
If you had known all the facts at the time, would you have done things differently?

Commas in the third conditional

Like with the other conditional structures, we can reverse the order of the clauses with no change in meaning.

If you had closed the door, the dog wouldn‘t have escaped. = The dog wouldn’t have escaped if you had closed the door.

When the if-clause comes first, it is followed by a comma. When the order is reversed, we do not use a comma.

If we had stayed a minute longer, the lion would have attacked. (comma after if-clause)
The lion would have attacked if we had stayed a minute longer. (no comma)


Remember! If and would never appear in the same clause:

If I had known he was a criminal, I never would have married him.
not: If I would have known …

We often use the modal verbs could and might instead of would in the third conditional.

If you had asked me, I could have helped you.
He might have answered if you had called him earlier.