The first part of this grammar deals with real conditions, sometimes known as "zero" and "first conditional" sentences. Part II of this grammar -hypothetical conditions referring to present or future time- will be studied in Unit 4. Finally, Part III -hypothetical conditions referring to past time- will be studied in Unit 8 of this course.

WHEN DO YOU USE A CONDITIONAL SENTENCE?

When you want to talk about a possible situation and its result, you use a conditional sentence:

If people invite you to a party, they expect you to come properly dressed.
If I wear the green jacket, it'll go with my new shoes.

In this kind of conditional sentence there is a real possibility that what is described will happen. There are two main types:

You use this type of conditional sentence to talk about something that is always true. It is like a law. In this type of condition if has a similar meaning to when:

If people invite you to a party, they expect you to come properly dressed.
(= When people invite you to a party, they expect you to come properly dressed)

If you heat ice, it melts.
(= When you heat ice, it melts)

You use this type of conditional sentence to talk about a probable future result:

If I wear the green jacket, it'll go with my new shoes.
If you don't study, you won't pass the FCE exam.

In these conditional sentences we can usually use unless instead of the structure if... not:

Unless you study, you won't pass the exam.
(= If you don't study, you won't pass the exam)

Unless I wear clothes like theirs, my college friends will think I'm a bit strange.
(= If I don't wear clothes like theirs, my college friends will think I'm a bit strange)

The examples above show two patterns for this kind of condition. But other combinations of verb forms and modal verbs are possible. Here are some of them:

If you get the chance, visit Delphi.
If you have finished, you may go.
If you're feeling ill, you ought to see a doctor.
If she can do that, she can do anything.
If you're not nervous, why is your hand shaking?