The Third Conditional

The Third conditional is used:

If you start with the if clause then you need to separate it from the main clause with a comma.

If clause	Comma	Main Clause
If + Past Perfect	,	Subject + would + have + past participle

If you start the sentence with the main clause you must not separate it from the If clause with a comma.

Main Clause	If Clause
Subject + would + have + past participle	If + Past Perfect

For instance:

If clause	Comma	Main Clause
*If + Past Perfect*	,	*Subject + would + have + past participle*
If I *had studied* harder	,	I *would have passed* the exam.
If I hadn’t eaten so much	,	I wouldn’t have felt sick.

Main Clause	If Clause
*Subject + would + have + past participle*	*If + Past Perfect*
I *would have passed* the exam	if I *had studied* harder.
He would have been on time for the interview	if he had left the house at nine.

Example	Explanation
If you had driven more carefully, you would not have had an accident.	Criticism: You had an accident because you didn’t drive carefully enough.
If we had played a little better, we could have won the game.	Regret: We didn’t play well, so we lost the game.
If you had saved your money, you could have bought a computer.	Criticism: You didn’t save your money, so now you can’t afford a computer.
If it had snowed, we could have gone skiing.	Regret: It didn’t snow, so we couldn’t go skiing.

This is the way we imagine how things could have been different in the past. If something had been different, something else would have happened.