The river is so deep that the children can't swim in it.——The river is too deep for the children to swim in .(从句中的it要去掉),这里的“从句中的it要去掉”怎么解释,原来的句子不是从句吗?

从句的意思是后面跟着的是一个完整的句子，即有主语谓语宾语这么一个句子。 so... that...that引导的就是一个完整的句子，所以这里里有个it.如果没有it,that 后的句子就显得不完整。
too...to..是一个句型而不是一个完整的句子。 一般学校老是会说too...to do sth.如果改一下，children swim in the river.对国人来说像这样是个正常的顺序，而你的第二个句子，river从正常语序的宾语变成了主语，成了所谓的倒装结构。为避免麻烦累赘就不需要再使用it了

The river is too deep for the children to swim in 这个句子就成了一个简单句了
The river 主语
is 系语
too deep 表语
for the children 目的状语
to swim in 动词不定式做表语补足语

The river is so deep that the children can't swim in it.此句中的that引导一个【原因状语从句】。
The river is too deep for the children to swim in .【这里的是too... to不定式结构】，不定式to swim in做结果状语。不定式与The river是【动宾关系】。
又如：
The room is too small for so many of us to live in.

The river is too deep to swim in （我把for the children省略，更易接受）这不是从句，是单句，to swim in 的宾语作为主语提前了。
that the children can't swim in it为从句，故介词宾语it不能省略。

The river is so deep that the children can't swim in it.——是原因状语从句,所以从句中要加it,构成the children can't swim in it的完整句子.
The river is too deep for the children to swim in.---- 是简单句,所以不加it,再如：
I need a chair to sit on.
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