There is no contradiction here. Point 1 My book says "must" is only used in the present tense, but not in the past or future tense. Correct. But note that this is not a convention: it's a fact. The fact is that the modal verb 'must' only EXISTS in the present tense: like all modals, it cannot be combine with 'will', and unlike other modals, it doesn't even have a past form. It is therefore IMPOSSIBLE to use in any other tense. Point 2 A: "Must he go to America tomorrow?" B: "No, he doesn't have to." This dialogue in the PRESENT tense. 'Must' is present tense, and 'He doesn't have to' is present tense. The word 'tomorrow' does not change the tense: it tells us that we are talking about future TIME, but the tense ( in other words, the verb form) is present tense. In fact, we often use present tense to talk about other times - it's very normal to refer to future events, especially fixed future events, in the present continuous or present simple. For example, 'I'm going to America next month' or 'My plane leaves at 10 am tomorrow'. Do not confuse 'time' and 'tense' - 'tense' only refers to verb forms. I hope that makes sense. Now I'll tell you what's wrong with the dialogue in your book: it's not real English. Some very outdated textbooks, particularly those used in China and Russia, teach the interrogative construction 'Must he...?'. However, the reality is that this question form is not used in modern English. It died out about 50 years ago! So, while the dialogue is technically correct in grammatical terms, it sounds very unnatural. In real English - that's to say, modern native-speaker English as opposed to the strange, stilted language you find in old textbooks - the dialogue would go like this: A: "Does he have to go to America tomorrow?" B: "No, he doesn't have to."

'Must he go to America tomorrow ?' is actually a present tense. It is referring to his situation now, it is asking if there is a requirement for him now to go to America tomorrow. It is the requirement to act, not the action itself, that is in the present

Thank you for your reply.

Definitely Correct🌟