请就这道题给我讲一下独立主格结构.It__ a hot day,I decided to go for a swim.此处为何不能填is?而只能用being?

问题描述:

请就这道题给我讲一下独立主格结构.It__ a hot day,I decided to go for a swim.此处为何不能填is?
而只能用being?

这个 就相当于 For it is a hot day。。。。

填is,前后为两个独立的句子,两独立的句子,前后必须有连词连接,而本句无连词.