甚麼時侯do you問問題?

你想知對方現在的想法或想做的事時便用do you ...?
Do you want to have some ice-cream?你想(現在去)食雪糕嗎？

問對方過去發生過的事情或感受便用did you ...?
Did you have ice-cream yesterday?你昨天有沒有食雪糕？

In the exam.room or not for a student:-
"do" is a verb used to refer to actions that you do not mention by name or do not know about;in this case questions:-
What are you doing this evening? Are you doing anything tomorrow ?
What can I do?
"Do you?" in the simple present tense:-is used for a habitual, permanent or repeated action,eg:-
Do you come to this class every day?Do you speak English? Do you always sleep with your air-con on? Do you smoke as a student?
Also for a general statement, or a proverb, where no particular time is thought of,eg:-Do actions speak louder? Does Victoria Harbour separate HK and KLN ?Do his family come from England? Does Confusius say something?
Also for Time or Condition; for Summary; for a future action about a decision; for exclamatory etc.
"Did you?" in the simple past tense is used to express an action completed at some point in the past,eg:-
Did you arrive at our sc.hall yesterday? Did you live at Kowloon Bay? Did you went to the tutors last night? The simple past tense used yesterday, last week, in 1997, when about time, eg:-When did you go there alone? What time did you arrive at school?
"Did you?" used in conditional sentences, expressing a supposition,eg:-Did you worked hard to pass the exam? Did you accept his offer? Did you want to make trouble? Did you ask and cheat in exam?
Other action completed tense be used on :-succeed/progress , task/activity, study, solve, make, copy, finish, spend time, deal with, cheat, punish, take drugs in school, etc.Hope you're successful.