need什么时候用情态动词,后面加动词原型.

need doing 是用v-ing来表示被动,某物需要被怎样,比如 The desk needs repairing.
need to do 是是主动,主语常是某人,某人需要做什么,比如 I need to do the homework now.
以上两种情况下need都是行为动词.
need做情态动词时后面不跟不定式而是直接跟动词原形.
最主要的区别是：need表示主动含义时,肯定句中常做行为动词,也就是加to do. 如: I need to do my homework now.
否定和疑问句中常做情态动词,加动词原型,否定直接加NOT, 疑问直接提前.
如： Need I tell him the new?
You need not tell him the news.