用Yes, sb.must.
例如 Need I do sth.?
Yes, you must.
No, you needn't
下面是某语法书上的内容
对情态动词Need引导的问句的几种回答形式：
Need I finish the work today?
----Yes, you must./ have to.
Yes, you ought to/ should.
No, you needn't.
No, you don't have to.