The if-clause is used when you want to express that something will happen/would happen/would have happened because something else has or has not occured before. It consits of a main-clause and an if-clause.

I. clause: likely to happen

If Ron becomes Quidditch captain, he will buy a new broom.

( Falls Ron Quidditch Kapitän wird, wird sich einen neuen Besen kaufen.)

II. clause: not very likely to happen

If Harry went to bed earlier, he would be fitter at Quidditch practise.

(Wenn Harry füher zu Bett gehen würde, würde er beim Quidditch Training fitter sein.)

III. clause: in the past, not going to happen anymore 

If George had learned for the test, he would have got a better grade.

(Wenn George für den Test gelernt hätte, hätte er eine bessere Note.)

There is also a mix type between the second and the third clause:

If Voldemort had not killed Harry´s parents, they would still be alive now.

(Wenn Lord Voldemort Harrys Eltern nicht umgebracht hätte, wären diese jetzt noch am Leben.)

Signal word: now

Zero conditional sentence: something that happens under certain circumstances

If you don´t wear gloves, your hands will freeze.

(Wenn du keine Handschuhe trägst, werden deine Hände einfrieren.)

In some cases you can interchange the "if" with "when" or "whenever".

Send a letter to Professor Dumbledore, whenever (instead of "if") there is an issue with the leprechauns.

(Schreibe einen Brief an Professor Dumbledore, wann immer es ein Problem mit den Gnomen gibt.)

You can also start with the main-clause, the meaning stays the same for example in :

Ron would have got a better grade, if he had learned for the test. 

Always remember there is no "would" in the if-clause! The only exception is "If you would be so kind..."