请解释一下这个语法？

Question

we cut down the number of people who needed to be involved in important decisions.为什么need要加ed?它是从句的谓语动词吗？

Answer 1

need 如果作“情态动词”用时，没有人称和时态的变化 ，直接与not连用构成否定句。

MODAL Need is also a modal. 必要 (用于劝告或建议)

•  "I'll put the key in the window."—"You needn't bother," he said gruffly.
“我将把钥匙放进窗户里去。”—“不必麻烦，”他粗声粗气地说。

•  Look, you needn't shout.
喂，你不必喊。

MODAL Need is also a modal. 一定 (用于劝告或建议)

•  You needn't worry.
你不必担心。

•  We have learned that a market crash need not lead to economic disaster.
我们已经认识到一次市场崩溃未必导致经济灾难。

need 也可以用作“实义动词”，后接名词、代词、不定式作宾语；此时，有人称和时态的变化，要用do not（does not）来构成否定句。

•  Well, for Heaven's sake, you don't need to apologize.
好啦，看在老天爷的份上，你不必道歉了。

•  He replied, with a reassuring smile, "Oh, you don't need to worry about them."
他面带鼓励的微笑答道：“哦，你不必为他们担心。”

只要看到needs..., needed..., need to do... 就可以判断出：此时need是用作实义动词，而不是情态动词。

Answer 2

We cut down the number of people who needed to be involved in important decisions.
我们减少了需要参与重大决策的人数。
这句话是一般过去时。needed是who引导的定语从句的谓语。

Answer 3

不是谓语动词

Answer 4

we cut down the number of people 主句时态就是一般过去时；cut动词形式不变；
定语从句who needed to be involved in important decisions，谓语动词needed自然用过去时。