The subordinating conjunctions if and whether can both be used to introduce indirect Yes / No questions.
- I don’t know if she will come. OR I don’t know whether she will come.
- I asked if she was interested in the offer. OR I asked whether she was interested in the offer.
In some cases, whether is preferred to if.
After a preposition
Only whether is possible after a preposition.
- Incorrect: We haven’t settled the question of if we should move into a new house.
- Correct: We haven’t settled the question of whether we should move into a new house.
Only whether can be used before to-infinitives.
- Incorrect: They can’t decide if to wait or leave.
- Correct: They can’t decide whether to wait or leave.