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When and If

Basics

When refers to a time: it means 'at the time'.

I will be on the plane when it leaves. (At the time that the plane leaves, I will be on it.)

If refers to a possibility. Like the German falls, it means 'in the event' or 'under the possibility'.

I will tell you if anything happens. (In the event that anything happens, I will tell you.)

The choice of when or if is important in English because it distinguishes between something which will definitely happen (when) and something which may or may not happen (if):

I will tell Sid when I see him. (I will definitely see Sid, so he will definitely be told.)

I will tell Sid if I see him. (I may or may not see Sid, so he may or may not be told.)

If you get this wrong, it causes real problems in understanding. In Switzerland, my German boss could not use the word if: he always said when. He used to say to me

Only do this when you have time.

I had to try to work out if he meant

Only do this if you have time. (It is possible that you will not have time: in this case, do not do it.)

Only do this when you have time. (Eventually you will have time to do this: do it then.)

Other Forms

If, but not when, is also used as a synonym for whether (German ob).

Correct: We will see if you are right.
Correct: We will see whether you are right.
Error: **We will see when you are right.

When is wrong here because this is a possibility, not a time.

When could be used in a similar context for a time:

We will see when the packages arrive. (This assumes that the packages will definitely arrive, but we want to know at what time they will arrive.)

We will see if the packages arrive. (We do not know whether the packages will arrive or not.)

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