Grammar : Articles

Indefinite Article
A/an are indefinite articles. We use them with singular nouns. it shows that the noun is general, not specific.
Examples:

• A man ('seseorang', 'seorang',  NOT 'orang itu')
• An ant ('seekor semut', NOT 'semut itu')
• a table ('sebuah meja', NOT 'meja itu')

A is used when the initial sound of the word is a consonant.
Examples :

• a book
• a cat
• a university (exception)

An is used when the initial sound is a vowel. 
Examples :

• an hour
• an onion

Definite articles 
The is definite articles, we use the with a singular or plural noun. It shows that the noun is specific, NOT general. Look at some examples :

• The book is useful.
• She is sitting in the middle of the room.
• He is playing the piano.

(Taken From :English On Sky 2, 2007)