if / when / whether / whether or not

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 1  Use if to express a condition.

Use when for situations that require preparation. Also use when to denote the passage of time.

Use whether to express uncertainty and alternatives. Don’t use whether or not in this case.

Only use whether or not if you want to emphasize that there are two possibilities or if you mean under any circumstances. However, it’s often better to rephrase the sentence in this case.

 2  Don’t use then in if-clauses. It’s a needless filler word and can even be misinterpreted in the sense that readers think that there’s a time-based relation.

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Examples    1 

Yes:

If the engine starts ….

(means: It’s not clear whether the engine will start.)

Yes:

When the engine starts ….

(means: At that point in time when the engine starts.

Yes:

If you need help, read the manual.

Yes:

When Setup is complete, restart your computer.

Yes:

To find out whether you need a new ink cartridge, ….

Yes:

You must pay your taxes whether you want to or not.

Top:

You must pay your taxes even if you don’t want to.

Examples    2 

No:

If you need help, then read the manual.

Yes:

If you need help, read the manual.


if / in case

If-clauses