比较下面两句：
What we need is more time.
What we need are doctors.
为何一句用单数，一句用复数？
赞同曹老师的解答。
看来xiaowuya520网友对主语从句的主谓一致还不甚知道，在此引用章振邦《新编英语语法概要》中的一个章节供参考：
4.5 A wh-分句和that-分句作主语
所谓wh-分句，就是由wh-疑问词（what, who, why, how, whether）引导的名词性从句。这种分句作主语，其谓语动词一般用单数。比如：
What caused the accident is a complete mystery.
Who was responsible for the accident is not yet clear.
Why she did this is not known.
How the book will sell depends on its author.
Whether she comes or not doesnt concern me.
两个由and连接的名词性分句作主语，假如表示两件事情，其谓语动词用复数。比如：
What caused the accient and who was responsible for it remian a mystery to us.（导致事故是什么原因什么，哪个应付事故负责任，对大家来讲还是一个谜。）
在以what-分句作主语的SVC结构中，假如主语的补足语是复数名词，主语的谓语动词也可用复数，。比如：
What they want are financial aids.（经济援助）.
What was real to him were the details of his life.
假如what分句的动词和补语都是复数，主句的谓语动词也可用复数：
What are often regarded as poisonous fungi（有毒的“真菌”）are sometimes safely edible（可以安全食用）。
What appear to be large windows in the second story are glass heat collectors（玻璃集热器）.
当what-分句是一个带有复数意义的并列结构时，主句谓语动词用复数：
What I say and think are no business of yours.（我说的和我想的不关你的事。）
在这里，what I say and think = what I say and what I think,指的事两个离别的过程，所以主句用复数。
名词性that-分句作主语一般用于先行it结构。比如：
It was obvious that the driver could not contrl his car.
It is sheer luck （纯然运势）that he is still alive.
但在正式语体的肯定上下文中，为了强调that-分句的内容也可将that-分句直接置于主语的地方。
比如：
There is not the remotest possibility of anbyones calling upon me, and that I should call upon anyone else is a thing undreamt of.（有人访问我的可能性极小，而我去访问其他人则是做梦也没想过的事。）
由上述诸例可以看出，无论是置于句首或句末，以that-分句作主语，主句谓语动词一般用单数。
