一般情况下的, 回答如上:
就是: Yes,you must.
No,you don't have to/needn't .
但是有一个特殊情况, 就是假如must在句中表达肯定的猜测推断的时候, 有例外, 这时候就要看句子暗含的时态.
例句: He must be in the classroom (now), isn't he?
他现在肯定是在教室, 不是么?
He must have been to Beijing (last year), didn't he ?
他去年肯定是去过北京了, 不是么?