Diagramming sentences
a and an are indefinite articles -- Use an before a pure vowel (not "universe" yu sound)

An article is one kind of determiner; there are only three articles-- a, an, the.
Other determiners are demonstratives (of which there are only four: this, that, these, those) and quantifiers, which give definite or indefinite numbers or quantities. There are many quantifiers-- all, some, many, few, seventeen, half etc.

has a reference sheet.