1) If + Present Tense ...... Will + Infinitive / Present Simple / Imperative
ex: If he studies hard, he will pass the exam.
sau : He will pass the exam, if he studies.
2) If + Past Tense ......... Would + Infinitive
ex: If I had more time, I would help you.
sau: I would help you, if I had mor time.
3) If + Past Perfect Tense ..........Would + Have + Past Participle
ex: If I had studied, I would have passed the exam.
sau: I would have passed the exam, if I had studied.