There are exactly three articles: A, AN, and THE.
The only difference between the three are whether they are Definite (the) or Indefinite (a, an) articles. To know which one to use, you just need to determine whether or not the object is a definite thing or not.
For example, if you are talking about a specific fence, you would use “the.” If you are talking about any fence, you would use “a.”
It’s that easy. Most of the time, you can easily tell which to use and have no problem with them. If there is trouble, it is choosing between A and An. Just remember, use A before non-vowel sounds and An before vowel sounds.
On an interesting note, there are many words that changed because of their article. For example, the word “nickname” used to be pronounced “ickname” (probably from the German “Ich” meaning “I” and “name”). Anyway, you used to say “an ickname,” but that quickly changed to “a nickname.” Cool, huh?