He, she or it take does; I, you, we, they take do; past takes did for everyone: Where does she work?
Skip do with be: I'm not ready.
Short answers reuse the question's form: Yes, I do; be and modals answer themselves.
Emphasis: He did call you; full forms like did not sound formal.
If your sentence asks about someone else's situation and starts with a wh- word that isn't the subject, put do, does or did between the wh- word and the subject: Where do they live? tells your reader you're asking about their home, not reporting one.
Match the form to the subject: does with he, she or it, do with I, you, we and they, and did for everyone in the past. So Where does she work? but Where do they work? and Where did she work last year?
If you're asking Why didn't you call me? about a friend who missed your message, didn't does two jobs at once: it makes the question past tense and negative, so call stays plain. Without did, Why you not call me? isn't standard English.
If you're saying what didn't happen and your verb is plain present or past simple with no other auxiliary, use do plus not: I don't eat meat is correct because eat has no negative form of its own.
If the thing that didn't happen involves be, skip do entirely: She isn't coming and They weren't happy are already negative, and She doesn't be coming would be wrong. Some main-verb uses of have also take not directly, so I haven't the time can appear in formal British English.
If you're telling someone casually what didn't happen, the short forms don't, doesn't and didn't are the normal choice: I don't eat meat sounds conversational, while I do not eat meat comes across as formal or emphatic written English. That formality gap also shows in negative questions: Why did she not phone? sounds more formal or forceful than Why didn't she phone? So pick the full form when you want your wording to carry weight or a formal register, and the n't form for everyday talk or writing. Short answers borrow the same form you chose for the question: Yes, I do answers Do you like it?, and No, she doesn't answers Does she know? That's why the answer's form changes with the question's tense and subject, not with the answer's own subject alone. One boundary to watch: when be or a modal asked the question, the answer borrows that verb instead of do, so Is she coming? takes Yes, she is, and Can you swim? takes Yes, I can.