The third conditional is used to talk about a situation that didn’t happen but we imagine the result. (eg. If I had known, I would have checked twice. —> I didn’t know, but if I did, I would have checked twice. or I would have checked twice I had known.)

How to form the 3rd conditional :

– Affirmative form : Condition + Subject + past perfect, Subject + would have + past participle

– Negative form : Condition + Subject + past perfect (not), Subject + wouldn’t have + past participle (eg. If I hadn’t known, I wouldn’t have checked twice).

– Question form : Condition + Subject + past perfect, would + Subject + have + past participle
(eg. If I had known, would I have checked twice?).

We can also use Could and Should :

– I should have checked twice. (Means that something was a good idea but you didn’t do it)
– I could have checked twice. (Means you had the ability to do something but you didn’t do it)