Question

1.what we need is time 2.what we need are good doctors 这两句话为什么用what，what做什么成分

为什么不用that

Answer 1

解析：What we need is time. 与What we need are good doctors.
1、两个句子均为主语从句，what均作need的宾语。
2、两个句子都不能用that，因为we need后缺少宾语，即主语、谓语、宾语结构的完整句子，没有宾语，此处可以理解为：We need something. 由what来代替something。
3、我们可以将两个句子改为：
All that we need is time. 或All that we need are good doctors.
此处need的宾语为all，亦可以用something等

Answer 2

你好。在你的句子裏面，what是time和doctors的表语。
1.what we need is time
== Time is what we need.
what == something

2 what we need are good doctors .
Good doctors are what we need.
what== people whom / the people that

3 用what,不用that是因为表语部分没有名词可以指代。