Modern English (sometimes New English or ne (ME)

Did you know that all articles are determiners, but what exactly is a determiner? Find out here.
3. Indefinite articles are used to refer to things that the speaker and listener don’t know the identity of.

• 
  Vick has a dog. (We know Vick owns a dog, but we don’t know anything about it.)
  
• 
  A woman is standing outside. (We don’t know who the woman is.)
  

In writing and speech, indefinite articles are often used to introduce the first mention of a noun. Once the noun has been introduced, it is then possible to use a definite article.

• 
  A truck drove past. The truck was huge. (The noun truck is introduced with the definite article a. After it is introduced, we can use the to refer back to the same truck.)
  

Unlike definite articles, it is considered grammatically incorrect to use indefinite articles with plural nouns.