Difference between whether and if

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.

Before to-infinitives

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.