9 indirect questions
Whether and if can both introduce indirect questions.
I’m not sure whether/if I’ll have time.
I asked whether/if she had any letters for me.
After verbs that are more common in a formal style, whether is preferred.
We discussed whether we should close the shop.
(More normal than We discussed if…)
In a formal style, whether is usually preferred in a two-part question with or. The Directors have not decided whether they will recommend a dividend or reinvest the profits.
If an indirect question is fronted (see sentence structure 5: fronting), whether is used.
Whether I’ll have time I’m not sure at the moment.
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0 prepositions
After prepositions, only whether is possible.
There was a big argument about whether we should move to a new house.
I haven’t settled the question of whether I’ll go back home.
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1 infinitives
Whether, but not if, is used before to-infinitives.
They can’t decide whether to get married now or wait, (NOT – if to get married …)
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2 subject, complement and adverbial clauses
When a question-word clause is a subject or complement, whether is normally preferred.
Whether we can stay with my mother is another matter, (subject)
The question is whether the man can be trusted, (complement)
The question is if… is also possible, but less common.
The question is if the man can be trusted.
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3 not used in echo questions
If and whether are not normally used in ‘echo questions’ (see questions 4: echo questions).
Are you happy?whether and ifAm I happy? No! (not .. .-If/Whether I’m happy?-…)
For more information consult a good dictionary like Langman, Oxford, Merriam Webster, or Collins.