I'm just curious about that problem in the title.
Which form is correct?
I've checked in English grammar that in simple past tense when using negative form
you should put first didn't and then after this the verb in infinitive.
But it sounds a little bizarre for me.
Welcome to the forums.
"Didn't see" is correct. The rule might sound bizarre to you, but that's the way it is.
In another context, using the same rule, "didn't saw" would be correct. "I didn't saw the legs off the table", for example.