need在什么情况下,做情态动词,（除了选择题外,怎么判断）给你一个题，让你在need后面填一个动词，怎么分辨是填原形还是不定时？

need 看后面动词的形式啊！情态动词后加原形，不然用to do/doing,need sb to do sth

1．need作情态动词无人称或数的变化,后接动词原形,多用于否定句和疑问句中.如：You needn't worry.你不必担心.
2．由need引出的一般疑问句,肯定回答常用must或have to;否定答语常用needn't.如：
一、need作情态动词的用法：
need用作情态动词时表“必须必要”的意思,后直接跟动词原型,且need无人称、数的变化（第三人称单数不加s）,也没有非谓语形式（不定式、分词）.
注意:
need用作情态动词时常用于否定句和疑问句中.
例：Need he bring his laptop tomorrow?明天他必须带他的笔记本过来吗?
Come on,you needn't worry about it becuse it's not your fault.行啦,你不必担心的因为那不是你的错.
She needn't have arrived so early,need she?她本不必到得这么早,不是吗?
注意:
must引导的一般疑问句的否定回答必须使用needn't.例如：
-Must I leave?我必须离开吗?
-No,you needn't.不,你不必离开.
二、need作实意动词的用法：
1、need + something 需要某物
例：I need some fruits to eat.我需要些水果来吃.
2、need to do something 需要做某事
例：You don't need to have the bike repaired right away.你不必马上去修车.
3、need doing = need to be done 需要……
例：The room needs cleaning = The room needs to be cleaned.房间需要打扫了.
4、need somebody to do something 需要某人做某事
例：I need someone to help me out of this problem.我需要找人来帮助我解决这个问题.
三、need作名词的用法：
need用作名词时,既可以是可数名词,也可以是不可数名词.主要常用句型如下：
1、(There's) no need to do something 没必要做某事
例：There's no need to cover such a long distance.没必要走那么远的距离.
There's no need for me to break the window because I have the key.我没必要打破窗户因为我有钥匙.
No need to run since we're not pressed for time.不必跑啦,我们 又不赶时间.
也可以单独使用：There's no need.意思是“没有必要”.
2、in (great) need of （非常）需要
例：The rescue team is in great need of medicine.救援小组非常需要药品.
3、if need be 如果需要的话
例：If need be,I'm going to do you a favor.如果需要的话我会帮你的.
四、关于need的一些短语、俚语用法.
1、A friend in need is a friend indeed.患难之交才是真朋友.
二、作行为动词
need 作为行为动词有人称和数的变化,后面可接名词、代词、动名词及带to 的动词不定式；可用于肯定句、疑问句和否定句中,如：She needs help.她需要帮助.I don't need to see the doctor.我不需要看病.—Do you need to go at once?你需要马上走吗?