need什么时候做情态动词,什么时候做实义动词?如何区分?

need v.
做实意动词时,有两种用法：
1.need to do sth.需要做某事
e.g I need to study hard for th exam.我需要为了考试努力学习.
He needed to drink some water.他需要喝些水.
2.need doing sth.需要被做某事（=need to be done sth.）
e.g The flowers need watering every day.花需要每天（被）浇水.
=The flowers need to be watered every day.
The classroom needs cleaning.教室需要（被）打扫.
=The classroom needs to be cleaned.
做情态动词时,通常只用疑问式或否定式.
其否定式常用于回答Must 的疑问句
e.g -Must I finish it today?
-No,you needn't
其疑问式用于构成疑问句
e.g -Need I wear a coat?
-Sure,it's cold.