need做情态动词时没有时态的变化,过去式也是need.后面接动词原形.如：
I need write a letter yesterday.
Need he answer it then?
Yes,he need.
No,he needn't.
情态动词need表示对过去的推测时常和现在完成时连用.如：
You needn't have hurried.你当时不必这么匆忙.
need做实义动词（行为动词）时,过去式是needed.后面接to do.如：
I needed to write a letter yesterday.
Did he need to answer it then?
Yes,he did.
No,he didn't.