Whether and If
We can generally use both whether and if to introduce indirect yes/no questions.
- I am not sure whether/if she will come.
- I asked whether/if she had any letters for me.
- I don’t know whether/if I can come or not.
Cases where only whether is possible
After prepositions only whether is possible.
- There was a big argument about whether we should move to a new house. (NOT There was a big argument about if …)
- I haven’t settled the question of whether I should settle abroad.
Before to-infinitives, only whether is possible.
- They can decide whether to get married now or wait.