Use if to introduce a condition, as in If the library closes early, use the return box. Both if and whether can introduce many indirect yes-or-no questions. I asked if the library was open and I asked whether the library was open report the same question.
Use whether in these patterns
- Before an infinitive. We’re deciding whether to renew the book. You can also use whether at the start of a sentence, as in Whether the library opens on Sunday depends on staffing.
- After a preposition. Our plans depend on whether the library is open.
- Directly before or not. Tell me whether or not the library is open. With or not at the end, Tell me if the library is open or not also works.
Avoid a double reading
Let me know if the library closes early could ask for a yes-or-no answer or for a message only in the event of an early closure. Let me know whether the library closes early asks for an answer either way. That small change removes the ambiguity.
