A practical english grammar - Vyssaja skola 1978

Indirect questions
Sound symbols

When questions are reported in an indirect way (not using the actual words of the person who originally said them), the following rules apply.

Yes-no questions. In indirect speech, yes-no questions (which have no question word in direct speech) are introduced by if or whether, and the subject and verb are in their ordinary statement word order.

Original question: Is John coming to the party?

Indirect question: Do you know if John is coming to the party?

Do you know whether John is coming to the party?

Find out if John is coming to the party.

If the main verb is past tense, then the verb in the indirect question is almost always past tense, too:

Is he here?

They asked if he was here.

Question-word questions. These questions retain the same question word they would have in their original form, and in addition, subject and verb are in statement word order, as in yes-no questions.

Why does he call you every day?

I don’t know why he calls you every day.

Why does he go to New York?

I wonder why he went to New York.

Many other changes are required in indirect questions, to make the pronouns, as well as the tense of the verb, correct.

When are you coming back from Europe?

I asked him when he was coming back from Europe.

He asked me when I was coming back from Europe.

Which do you prefer?

I don’t know which he prefers.

He didn’t tell me which he preferred.

He wanted to know which I preferred.