How to learn basic english?

"A" and "an" signal that the noun is indefinite, referring to any member of a group.

 For example:

• "My daughter really wants a cat for Christmas." This refers to any cat. We don't know which cat because we haven't found the cat yet.

• "When I was at the zoo, I saw an elephant!" Here, we're talking about non-specific thing, in this case an elephant. Maybe there were more elephants at the zoo, but in this case we're talking about only one.