need在什么情况下作情态动词 什么情况作行为动词

具体问题具体分析了，当表示“必须，不得不”是是情态动词；当表示“需要”时，是行为动词。

（1）情态动词，“必须，不得不”
You needn't talk so loud.
你不必这么大声讲话。
Need you go so soon?
你非得这么快就走吗？
You needn't have told him the news; he knew it already.
你不必再告诉他这个消息，他已经知道了。

（2）作行为动词
need
[ni:d]
（常与to连用）
需要
Plant needs water.
植物需要水。
He needed all his charm to persuade her that it was not his fault.
他需要使出浑身解数来叫她相信那不是他的错。
My shirt needs a button.
我的衬衫需要一枚钮扣。
She's on holiday; she needed a change from work.
她正在度假,她需要放下工作改变一下生活。
These six kitchens are all needed when the plane is full of passengers.
这六个厨房在飞机载满旅客时都用得着。
You didn't need to tell him the news; it just made him sad.
你无需告诉他这一消息, 那只会使他悲哀。

如果need后面可以直接接动词的话，它此时就作情态动词。比如：I needn't go to school tomorrow.

如果need后面接to do时，它此时就作实义动词。比如：I need to go to shcool tomorrow.

分析句子的结构，有动词的句子，NEED是情态动词，如果没有其他动词了，而且有需要的意思，那一定是动词。