need可以是情态动词，也可以是实意动词。

作为情态动词，need后接不带to的不定式，这时只能用于疑问句和否定句，不能用于肯定句。例如：

You needn't do it.

Need I come tomorrow?

但不能说 You need do it.

need作为实意动词，可以后接名词，也可以后接带to的不定式做宾语。既可以用于肯定句，也可以用于疑问和否定句。例如：

I need some help.

You need to come tomorrow.

You don't need to do it.

Do I need to do it?

当need后接动名词作宾语时，动名词的主动形式表示的是被动意义，相当于后接被动不定式。这时，句子的主语为something。例如：

The room needs cleaning. = The room needs to be cleaned.