,

* Zero conditional : If + simple present ==> simple present ( when it's always true) ex: If it rains, the sun doesn't shine

* First conditional : If + simple present ==> will + infinitive ( likely) ex: If it rains, I'll stay at home

* Second conditional : If + simple past ==> would + infinitive (unlikely) ex: If I won €1,000,000 , I would travel

* Third conditional : If + past perfect ==> would + have + verbe in the 3rd form (v3) (To refer to the past ==> you can't change it.) ex: If it had realized how awful it was going to be, I wouldn't have accepted it.