Whether and If

1. When both choices are given, we generally use whether rather than if:

• I don't know whether she's coming or not. (It would be possible to use if here, but less common.)

2. Before an infinitive with to we use whether:

• I don't know whether to accept their offer or not. (If would not be used here.)

3. We can use whether after a preposition:

• It's a question of whether we can agree on everything. (If would not be used here.)

4. Both whether and if can be used in indirect speech to introduce a yes/no question:

• Did they say whether/if they were going to be late?

5. After some verbs, we use whether:

• I doubt whether they'll make it.
• We discussed whether it was the correct decision.

Of whether | Of if | Whether to | If to
Dictionary.com definitions: Whether | If
BNC Online examples: Whether | If