Page 265 - Practical English Usage 3ed - Michael Swan, Oxford

256 if (1): introduction
1 uncertain events and situations
In clauses after if, we usually talk about uncertain events and situations: things which mayor may not happen, which mayor may not be true, etc.
Ask John ifhe's staying tonight. (He mayor may not be staying.)
IfI see Annie, I'll give her your love. (I mayor may not see Annie.) 2 conditions
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An if-c1ause often refers to a condition - something which must happen so that something else can happen.
Ifyou get here before eight, we can catch the early train.
Oil floats ifyou pour it on water.
Clauses of this kind are often called 'conditional' clauses. Verb phrases with would/should are also sometimes called 'conditional'.
'first', 'second' and 'third' conditionals; other structures
Some students' grammars concentrate on three common sentence structures with if, which are often called the 'first', 'second' and 'third' conditionals.
4
These are useful structures to practise. However, students sometimes think that these are the only possibilities, and become confused when they meet sentences like Ifshe didn't phone this morning, then she's probably away ('What's this? A fourth conditional?'). It is important to realise that ifis not only used in special structures with will and would; it can also be used, like other conjunctions, in ordinary structures with normal verb forms. For details, see the following sections.
position of if-clause
An if-c1ause can come at the beginning or end of a sentence. When an if-c1ause
'first conditional' if+ present
If we play tennis 'second conditional'
if+ past
Ifwe played tennis
'third conditional' if + past perfect
Ifwe had played tennis
will + infinitive I'll win.
would + infinitive I would win.
would have + past participle I would have won.
comes first, it is often separated by a comma.
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