English distinguishes among many different types of conditional sentences—that is, sentences that focus on questions of truth and that are introduced by if or its equivalent. The following examples illustrate a range of different conditional sentences. Each of these sentences makes different assumptions about the likelihood that what is stated in the if clause is true.

If you practice (or have practiced) writing frequently, you know (or have learned) what your chief problems are.

This sentence assumes that what is stated in the if clause may be true; as the alternatives in parentheses indicate, any tense that is appropriate in a simple sentence may be used in both the if clause and the main clause.

If you practice writing for the rest of this term, you will (or may) understand the process better.

This sentence makes a prediction about the future and again assumes that what is stated may very well be true. Only the main clause uses the future tense (will understand) or some other modal (59a) that can indicate future time (may understand). The if clause must use the present tense.

If you practiced (or were to practice) writing every single day, it would eventually seem much easier to you.

This sentence shows doubt that what is stated will happen. In the if clause, the verb is either past tense—actually, past subjunctive (32h)—or were to 1 the base form, even though it refers to future time. The main clause contains would 1 the base form of the main verb.

If you practiced writing on Mars, you would find no one to read your work.

This sentence imagines an impossible situation. As with the preceding sentence, the past subjunctive is used in the if clause, although past time is not being referred to, and would 1 the base form is used in the main clause.

If you had practiced writing in ancient Egypt, you would have used hieroglyphics.

This sentence shifts the impossibility back to the past; obviously, you aren’t going to find yourself in ancient Egypt. But a past impossibility demands a form that is “more past”: the past perfect in the if clause and would 1 the perfect form of the main verb in the main clause.