The clause after if, we usually talk about uncertain events and situations: things which may or may not happen/ be true. An if-clause oftern refers to a condition--something which must happen so that something else can happen.
There are three special structures named 'first', 'second' and 'third' conditionals.
First conditional If + present will + infinitive
If we play tennis I’ll win
Second conditional If + past would + infinitive
If we played tennis I would win
Third conditional If + past perfect would have + past participle
If we had played tennis I would have won
However, if-clause also has ordinary structures.
(1) The same tenses as with other conjunctions
When we are not talking about 'unreal' situations, we use the same tenses with if as with other conjunctions. Present tenses are used to refer to the present, past tenses to the past.
Oil floats if you pour it on water.
If John didn't come to work yesterday, he was probably ill.
(2) present tense with future meaning
We normally use a present tense to talk about the future.
I'll give him your love if I see her.
