I did submit my homework/I submited my homework?

"I did submit my homework" is emphatic. You should only use it to emphasize.
The normal way of saying it is:
I SUBMITTED MY HOMEWORK.
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An example of the first would be:
-Could you hand in your homework please?
- Oh, but I already submitted my homework.
- I haven't received it. Sorry, don't tell lies.
- But I DID submit my homework.
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As for the tense maybe you could have used the PRESENT PERFECT instead of the past simple because there is no definite time expression.

I did submit my homework. - emphatic, for example:
the teacher says: you didn't submit your homework.
so you say: I DID submit my homework.

I submitted my homework. - normal past tense

Some questions about these two sentences:
1) What is/are the difference(s) between these two sentences? (Meaning, grammar and tense)

2) Why do we need to add the word "did" in front of "submit"? (For the 1st sentence)

3) When should I use the 1st sentence and when should I use the 2nd sentence?
Or are they the same?

4) What grammar mistake(s) were found in this passage? (Let me know what I need to improve and what is wrong.)

Maybe somebody can solve my question. Thanks a lot!

P.S. I am a Hong Kong guy so please forgive my poor English. Thanks very much! Hope my English skills will improve with asking you guys questions!