OK Felix, this is an interesting question. We use both 'if' and 'whether' in indirect questions, so you could say, 'I don't know if she's coming', or, 'I don't know whether she's coming'.

But in some circumstances we can only use 'whether'. For example, before infinitives with 'to', we only use 'whether'. We say, 'I don't know whether to tell him', but you wouldn't say, 'I don't know if to tell him'.

We also only use 'whether' after prepositions. So we say, 'We had a long discussion about whether to go by car or by train'. We couldn't use 'if' in that sentence.

We also prefer 'whether' when the clause beginning with 'whether' is a subject or a complement. So you say, 'Whether you are agree or not makes no difference to me'. (The 'whether you agree or not' is the subject of that sentence.)

And lastly, we use whether with 'or'. So we say, for example, 'I didn't know whether I should laugh or cry'. Now some people who think that there are correct and incorrect ways of speaking English think that you shouldn't use 'whether' with 'or', but actually most people these days do use it.

And lastly, 'whether' does tend to be more formal than 'if' so then in the cases where you could use 'whether' or 'if', if you are speaking more formally, you would probably use 'whether'.

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