Second conditional is used to express: - unreal situations:

If I were 20 again, I would spend my nights out.

-things you don't expect to happen:

If I won on the lottery I would get nice cars and a big house.

Form is:

when the 'if'clause is in the simple past, the main clause goes with "would+ infinitive"

Third conditional is used when you want to express regrets/to criticize people: If only I had known.... I would have helped you.

Form:

'If'clause is in the past perfect tense and main clause is in past conditional(would+have).

Hope I helped.