How does one explain the difference in usage between "I am going to go get it." and "I will go and get it." Correct terms, please, to help a foreign student.

I am going = present progressive tense

I will go and get = future tense, compound verb

to get = infinitive form

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http://grammar.ccc.commnet.edu/grammar/verbs.htm
Scroll about half-way down for information on present progressive.

http://grammar.ccc.commnet.edu/grammar/future.htm

http://owl.english.purdue.edu/handouts/grammar/g_verbals.html#infinitives

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i need some help with my English homework

Of course! I'd be happy to help you with your English homework. What specifically do you need help with?