An Advanced English Grammar with Exercises - George Lyman Kittredge; Frank Edgar Farley

The following examples further illustrate the difference between these two constructions:—

1. I foresee the course which he will take. [Relative clause.]
I foresee which course he will take. [Indirect question.]
2. I heard what he said. [Relative clause. What = “that which.”]
I wondered what he said. [Indirect question. What is an interrogative pronoun.]
3. This is the man who brought the news. [Relative clause.]
The king asked who brought the news. [Indirect question.]
4. Here is a paper which you must sign. [Relative clause.]
The clerk will tell you which paper you must sign. [Indirect question.]

Note. In such a sentence as “Tom knows who saved the child,” the indirect question may at first appear to be a relative clause with an omitted antecedent (the man, or the person). If, however, we insert such an antecedent (“Tom knows the man who saved the child”), the meaning is completely changed. In the original sentence, it is stated that Tom knows the answer to the question, “Who saved the child?” In the new form of the sentence, it is stated that Tom is acquainted with a certain person, and to this is added an assertion about this person in the form of a relative clause.

445. An indirect question is sometimes expressed by means of an interrogative pronoun or adverb followed by an infinitive.

Whom to choose is a serious question. [Direct question: Whom shall we choose?]
John asked what to do. [John’s question was: What shall I do?]
I know where to go. [Direct question: Where shall I go?]
Tell me when to strike the bell.
I was at a loss how to reply.
I am in doubt how to begin this essay.

In the first four examples the italicized phrase is used as a noun (either as subject or object). In the fifth, the phrase how to reply is adverbial, modifying the adjective phrase at a loss.

446. The subjunctive was formerly common in indirect questions, and is still occasionally used after if or whether.

I doubt if it be true.
Elton questioned whether the project were wise.

447. The rule for shall (should) and will (would) in indirect questions is, to retain the auxiliary used in the direct question, merely changing the tense (shall to should; will to would) when necessary ([§ 442]).

I. Mere Futurity
1.	Direct:	What shall I do?
1.	Indirect:	I wonder what I shall do. You ask me what you shall do. He asks me what he shall do. I wondered what I should do. You asked me what you should do. He asked me what he should do.
2.	Direct:	Shall you lose your position?
2.	Indirect:	{I ask \| He asks} you if you shall lose your position. {I asked \| He asked} you if you should lose your position.
3.	Direct:	Will Charles lose his position?
3.	Indirect:	I ask if Charles will lose his position. {I \| You \| Tom} asked if Charles would lose his position.