Conditional sentences talk about a cause and a result or effect. However, they dont state the cause and result as ordinary facts. Instead, both the cause and the results may be probable events, possible events, unlikely events, or impossible events.
The cause is expressed in a clause that begins with if. The result is expressed in a clause that almost always includes a modal auxiliary:
If he hurries, he can still catch the train.
If you were right, then I’m wrong.
If we get rich, we’ll eat nothing but chocolate.
If I lived by the sea, I might learn to sail.
She would go back to college if she didn’t have any children.
If I won the lottery, I could buy you a house.
He would understand the problem if he were a woman.
As you can see, conditional sentnces can talk about the past, present or future. Conditional sentences can also talk about events that are likely and events that are only possible. All these differences -between likely and merely possible events, and between past, present, and future events- require different forms of the sentence’s verbs.
If something is CERTAIN or LIKELY TO HAPPEN if something else happens, we usually use the present tense in the if-clause and the future tense (with will) in the result or effect clause. (Conditional I).
If you give me the keys, I will start the van.
If he asks her, she’ll say no.
If the temperature reaches 300°, the whole thing will explode.
If the result or effect is only a POSSIBILITY, the if-clause uses the past tense, and the result clases uses would and the infinitive form of the verb. (Conditional II).
If he asked her, whe would say no.
If you wanted it, I would buy it for you.
To be COMPLETELY ACCURATE, the tense in the if-clause is actually the past subjunctive rather than the past tense. But the past subjunctive is identical to the past tense except in one verb (the verb to be), and today many native speakers ignore the diffrence completely.
Be in the past subjunctive always takes form were, even after I and in the third person singular, even many native English-speakers use was instead of were in such cases.
If I were working there, I would find another job. (Not I was)
If he were a little less lazy, his house would look a lot nicer. (Not he was)
If the side door were larger, we could move the piano through it. (Not the side door was even if it refers to “it”)
If we are talking about something that didn’t occur even though it was a possibility, we use the past perfect tense in the if-clause, and would have in the result clause. Conditional III
If they had come, they would have had fun.
He would have been hurt if he hadn’t moved quickly.