This is actually quite complicated, but I will give you the simplified version that is taught to people learning English as a second language. (Information adapted from A Basic English Grammar with Exercises by John Eastwood and Ronald Mackin, Oxford University Press 1988.)
When you use if you are usually starting a conditional clause — also called an adverbial clause of condition or an IF-clause. You do this to show cause and effect, or what MIGHT happen if something else happens. There are three main kinds of IF-clause:
1. Type 1: IF + simple present tense, then + will, can, may/might.
We use Type 1 to talk about future situations that the speaker thinks are PROBABLE.
We also use Type 1 but with PRESENT TENSE + PRESENT TENSE* to talk about things that are ALWAYS TRUE. Some grammarians call this the ZERO CONDITIONAL or Type 0.
2. Type 2: IF + simple past tense, then + would, could or might.
We use Type 2 to talk about future situations the speaker thinks are POSSIBLE but NOT VERY PROBABLE. We also use Type 2 to talk about UNREAL SITUATIONS in the present.
* “If I were rich” means I am not rich. We sometimes say “If I was rich” here, but careful writers will say “If I were rich.” The were here is not plural; it is what is left of the English subjunctive mood, which was the one you chose to express probability/improbability or certainty/uncertainty. If you study Latin or French long enough you will find the subjunctive more fully developed in those languages.
3. Type 3: IF + past perfect (or pluperfect) tense (had —–ed), then + WOULD HAVE, COULD HAVE or MIGHT HAVE.
We use Type 3 to talk about PAST SITUATIONS THAT DID NOT HAPPEN.
There is more to this, but that gives a good start.
* It is surprising how many native English speakers don’t know what they are doing when they make these changes; I know I didn’t until I had to teach newly arrived overseas students (adults) in a Language College way back in 1990! But the changes are important, as they actually change what the IF-sentence means.
For more on tense, go here.
For more on IF-clauses, go here, and keep going for a few more pages!
To test your understanding, go here. Then go here then click on “Conditional Tenses”, or try this quiz.
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