What's the difference of this two sentences?

[John76]

in conversation, you could use Did or Do. it really doesn't matter when having a casual conversation with someone. i think we're worrying too much about specific details and slight differences in the rules.

[Weibing]

We have no such thing as 'tense' in Chinese. Chinese students, however, is not unfamiliar with English 'tense' - they take much time to learn English grammar. So, even though they are not good at listening and speaking('dumb English'), when they sit for TOEFL exams, chances are that they will get a good score.

If you don't learn English 'tense', you can't truly understand what English-speakers say and write, and thus can't effectively communicate with them. What is more, when you take an English test, don't expect a good grade.

I can't imagine if I really don't know any grammar I could put any English here!

[TS]

As to 'present perfect', I have a question for you, what's the difference between the two sentences below?
1. I have finished my homework.
2. I finished my homework.

Present Perfect can be explained only in a paragraph of sentences. Please compare the following two examples.

Ex1: "Yesterday Daddy got two football tickets. I finished my homework. I may go with him now."
== Simple Past finished means at the same time span of the former Simple Past got. That is to say, I finished it yesterday.

Ex2: "Yesterday Daddy got two football tickets. I have finished my homework. I may go with him now."
== Present Perfect have finished means not at the same time span of the former Simple Past got. Present Perfect action happens in the Perfect Time between Yesterday and Now — this morning.

More details can be seen in the following link:
http://www.englishtense.com/newapproach/3_2_2.htm

Opinions are invited.