As with reported statements, the tense of the verb in the direct question often changes when it is turned to a reported question.
When the question is answerable by yes or no, use if.
Jennifer: “Do you have a reservation, Mr Thomas?” (direct questions)
Jennifer asked if he had a reservation (reported question)
When the question limits the answers by giving choices, use whether.
Simon: “Are you British or Australian?” (d. q.)
Simon asked whether she was British or Australian. (r. q.)
When the question is open to many answers, use the question word used in the direct question.
“What do you think of this plan?”
He asked me what I thought of the plan.
“Why did you choose our service?”
She asked why I chose their service.
Some modals also change when the direct question is turned to a reported question.
Will to Would:
Glenna asked me, “Where will you go?”
Glenna asked me where I would go.
Can to Could:
“Can you drop me off at the airport?”
She asked if I could drop her off at the airport.
May to Might:
“May I go with you? ” Sally asked Harold.
Sally asked Harold if she might go with him.
Must to Had to:
“Must you go?” Larry asked Bill.
Larry asked Bill if he had to go.