Conditionals are sentences made up of two clauses – an ‘if’ clause and a main clause – that are closely related. We use these structures to describe what will happen if we do a certain thing, or a certain event occurs. In other words, if a certain condition is true, a certain result will follow.
The ‘if’ clause tells you the condition and the main clause tells you what will happen as a result.
We use the 3rd conditional to talk about something in the past that did not happen. (Note that the 1st and 2nd conditionals refer to future events, but the 3rd conditional refers to a PAST event.)
The 3rd conditional indicates regret – you use this structure when explaining that you wish you had acted differently in the past. For example:
If I had known it was going to rain, I would have worn a raincoat. (You regret not taking a raincoat when you went out.)
I would have remembered to send John a birthday card, if you had reminded me. (You regret not remembering to send John a birthday card.)