The word 'whether' means 'if or not'. In some cases, they are interchangeable. For example, you can say: I don't know if he's coming to the party. I don't know whether he's coming to the party. These two sentences have the same meaning. They both mean that I don't know if he is or isn't coming to the party. However, you can't use 'whether' if your sentence doesn't include the meaning of 'yes/no' or 'if or not' . For example, in straightforward conditional sentences, you can't replace 'if' with 'whether' e.g. I'll read this report if I have time.

Use 'whether' only to offer a choice between 2 alternatives: whether {verb} THIS or THAT. For example: whether to choose this or that; whether to go here or there; whether to eat one or two... etc. Use 'if' for all other situations where you have to choose between them. Cheers, Old man.