[Grammar] Does this sentence make sence clearly?

[funfunny]

Today, while I was studing, I met a sentence like the following.

1. He is keeping quiet that he may not disturb his father.

I can't understand what the 'that' does in the sentence.

Does the sentence have the same meaning as the next two?

2. He is keeping quiet so that he may not disturb his father.
or
3. He is keeping quiet so he may not disturb his father.

I know when the so (that)-clause means a purpose, we can omit the 'that'.

Then, can I omit 'so' instead of 'that'?
Sentence 1, 2 and 3 are all correct and of the same meaning?

 

[Barb_D]

Welcome to Using English.

Thank you for using proper capitalization and puncutation in your post!

Your sentence as a whole reads awkwardly to me.
He is keeping quiet so [that] he won't disturb his father.

The "that" without "so" is awkward.
"that he may not" instead of "so he doesn't" is awkward.
Even "keeping quiet" doesn't sound quite normal. I'd say "being quiet."