0: If + present simple, present simple. If you heat wáter to 100 degrees, it boils. / 1: If + present simple, will + infinitive. If rains later, we’ll stay at home. / 2: If + past simple, would + infinitive. If I had a million dolars, I would buy a big house. / 3: If + past perfect,would + have+ past participle. If I had studied harder, I would have passed the exam.