Articles

That is probably one of the most difficult questions you could ask. Hmmm. Here goes...

If a determiner is supplied, you don't need an article. Determiners include numbers, words like "some", "those", etc., and possessives, such as "his", "John's", etc.

Proper nouns don't need an article, unless they're plural, in which case use "the".

Pronouns don't need an article, except in those extremely rare cases where they're being used as nouns! (e.g. "Rommie is a she").

Indefinite mass nouns don't need an article.

Indefinite plural count nouns don't need an article.

In all other cases, use "a" or "an" for indefinite; "the" for definite.

ANY of the articles ("a"/"an"/"the"/none) can be used to indicate genericity. In general, the article which makes least sense in any other context is used.

Rommie.

Not really - not without a more specific question. It's kind of like you've asked "What are the rules of grammar?"

Could you narrow down the question a bit?