When to use "whether" and when to use "if"

1. After prepositions, use only "whether."

   I haven't settled the question of whether I'll go back home.
   There was a big argument about whether we should move to a new house.

2. Use "whether," not "if," before infinitives.

   They can't decide whether to get married now or wait.

3. Use "whether" when a question-word clause is a subject or complement.

   Whether we can stay with my mother is another matter. - (Subject)
   The question is whether the man can be trusted. - (Complement)

4. Use "whether" if an indirect question is fronted.

   Whether I'll have time I'm not sure at the moment.

5. Use "whether" in a two-part question with or.

   The Directors have not decided whether they will recommend a dividend or reinvest the profits.

   Use "whether" after verbs that are more common in a formal style. We discussed whether we should close the shop.

6. You may use either "whether" or "if" to introduce indirect questions.

   I'm not sure whether [if] I'll have time.