If’ usually introduces conditional clauses. The same applies to other conditionals such as unless, should and in case which, however, perform other functions in different contexts.

If I drink milk, I get stomach ache. We use the first conditional to talk about things which might happen if an action takes place. We use if for things which are likely to happen and when for ...

Although there are quite a few different ways of forming conditional sentences there are common patterns known as zero, first, second and third conditionals. Refer to an imagined past result of ...