the teacher gave us so difficult a problem as most of us can't work out

问题描述:

the teacher gave us so difficult a problem as most of us can't work out
这里为什么要用as而不用that呢?

that后要加从句(完整句子),as再这里相当于which,指代problem
用that的话从句中要有it做宾语
即The teacher gave us so difficult a problem as most of us can't work out=The teacher gave us so difficult a problem that most of us can't work it out