Third Conditional

Introduction
The third conditional (also called conditional type 3) is a structure used for talking about unreal situations in the past. This page will explain how the third conditional is formed, and when to use it.
The structure of a third conditional sentence
Like the other conditionals, a third conditional sentence consists of two clauses, an "if" clause and a main clause:

if clause

main clause

explanation
If I had studied harder,

I would have passed the exam.

I failed the exam, because I didn't study hard enough.

If the "if" clause comes first, a comma is usually used. If the "if" clause comes second, there is no need for a comma:

main clause

if clause
I probably would have passed the exam

if I had studied harder.

We use different verb forms in each part of a third conditional:

if clause

if + subject + past perfect verb*
main clause

subject + would (OR could, OR might) have + past participle

*The past perfect is formed with the auxiliary verb "had", and the past participle (or third form) of the verb.

Note also that third conditional forms can be contracted:

Full form

If I had studied harder, I probably would have passed the exam.
Contracted form

If I'd studied harder, I probably would've passed the exam.

Using the third conditional
The third conditional is used to talk about things which DID NOT HAPPEN in the past. If your native language does not have a similar construction, you may find this a little strange, but it can be very useful. It is often used to express criticism or regret:

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.