A tall building has been sep up in what was a wasteland before.

Question

此处为何填what？不填which或that？what的用法总是弄不懂。唉

Answer 1

因为这里不是定语从句，而是一个宾语从句， what was a wasteland before.作介词in的宾语，what在这里作双重成分，一个算是in 的宾语，一个是was的主语，这是what引导主语从句的重要特征，再如
He cares much about what is important to him

Answer 2

这里是一个介词in + what引导的宾语从句。
A tall building has been set up in what was a wasteland before.
这栋高层建筑座落的地方曾经是一片荒地。

which和that引导的是定语从句，前面需要有先行词，如下：
A tall building has been set up in a place which/that was a wasteland before.本回答被提问者和网友采纳

Answer 3

what用作关系代词表示 所...的事物(或人)
We must decide what to do next.
我们必须决定下一步做什么。
I don't care what she thinks.
我不管她怎么想。

Answer 4

首席肯定不能用That,因为前面有介词in；再者，引导词what在前半句做宾语，在后半句要做主语，所以用what。