If he studies hard, he can pass the exam.

• We use the simple present tense "studies" in the if-clause to express a general condition.

If I were a police, I would arrest all criminals.

• This is a second conditional sentence, expressing an unlikely or hypothetical situation. We use "were" instead of "was" for "I" in formal English.

If I had had enough time, I would have helped you do housework.

• This is a third conditional sentence, expressing a past unreal condition. We use the past perfect tense "had had" in the if-clause.

If you heat ice, it melts.

• This is a zero conditional sentence, expressing a general truth or scientific fact. We use the simple present tense in both clauses.

It will become yellow if you mix red and green.