Question

It is such an interesting book that we all like it.

It is such an interesting book that we all like it. 为什么不是定语从句

Answer 1

因为从句we all like it中，主we谓like宾it都有，如果缺少宾语it，就是定语从句，即It is such an interesting book that we all like。
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Answer 2

你好。请比较用了什么词，少了什么词：
It is such an interesting book THAT we all like it. （结果状语从句）
It is such an interesting book AS we all like (定语从句)
又如
They are such good books THAT we will read them(结果状语从句)
They are such good books AS we will read.(定语从句)