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

If you practice (or have practiced) writing often, you learn (or have learned) what your main problems are.

This sentence assumes that what is stated in the if clause may be true; any verb 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 and again assumes that what is stated may turn out to be true. Only the main clause uses the future tense (will understand) or a modal that can indicate future time (may understand). The if clause must use the present tense.

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

This sentence indicates doubt that what is stated will happen. In the if clause, the verb is either past—actually, past subjunctive (23f)—or were to + the base form, though it refers to future time. The main clause contains would + 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. Again, the past subjunctive is used within the if clause, although past time is not being referred to, and would + 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 won’t 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 + the present perfect form of the main verb in the main clause.