so...that, so

[diamondcutter]

He spoke so fast that nobody could understand.
(From Practical English Usage)

He spoke fast, so nobody could understand.
(Rewritten by me)

The original sentence sounds like an exclamatory sentence while my version is much flatter in tone and it can be a reply to a why-question.

What do you think of my understanding?

 

[jutfrank]

Forget about the single word 'so' here. You need to learn so/such ... that ... as a specific and separate pattern. Have a look around the web to see if you can find some decent resources on this.

Here's a start: https://www.grammar-quizzes.com/8-8.html