if clause ---------------
simple present -------------
main clause
will-future / infinitive / modal + infinitive
If I study,I will pass the exams
I will pass the exams if I study.
i f clause -------main clause
simple past ----------------- would / could / might + infinitive
If I studied,
I would pass the exams
I would pass the exams.
if I studied.
if clause ----------------
past perfect---------------
main clause
would / could / might + have + past participle
If I had studied,
I would have passed the exams
I would have passed the exams.
if I had studied.