Conditional sentences contain a dependent clause beginning with if, when, or unless and can express generalizations, predictions, or possibilities. The three types of conditional sentence use different verb tense sequences.
Factual conditional sentences express related events that are habitual or governed by a law of nature. The verbs in both clauses are in the same tense - past or present - depending on the time of the action.
If it is a sunny day, I always go for a walk. If water is heated to 212 degrees, it boils.
Factual conditional sentences can also be used to make requests.
II you have finished copying my notes, please return them.
Predictive conditional sentences express future events that are planned or are likely to happen. The verbs in the two clauses are in different tenses: the present tense in the dependent clause, and the future tense in the independent clause.
If you don't study for that exam, you will get a poor grade.
Imaginative conditional sentences speculate about events that are possible but unlikely, that are completely impossible, or that never happened. The verbs in the two clauses are in different tenses. To talk about an event in the present, use the subjunctive were in the dependent clause, and use would, could, or might plus the infinitive form of the verb in the independent clause.
If I were you [but I'm not], I would marry that man.
To talk about an event in the past, use the past perfect tense in the dependent clause and the present perfect tense in the independent clause.
If Hitler had conquered Europe [but he didn't], the world would have been a much different place.
Do not confuse the imaginative conditional with a noun clause in an indirect quotation ("The instructor asked Karl if he was going to drop the class").