need 做情态动词的话是用need回答,如
-Need you see him?-
-No,I needn't./Yes ,I need.
need也可以用作实意动词,如,
-Do you need to see him?
-Yes,I do ./No,I don't .
must只是语气更强烈一点,need是需要,must是必须,need一定是对的