A/an and the are known as articles.  A and an are indefinite articles; the is a definite article. 

A and an are used only with and in front of singular countable nouns, but we can use the with any noun and when it is clear which one we mean.
Examples:
     A dog is an animal. (dog and animal are singular countable nouns.)
     The dog barking at John is my neighbour's dog. (It is clear which dog we mean.) 

When talking about permission, other than can, could, may, or might, we can also use be allowed to.
Examples:
     I'm allowed to stay up as late as I like.
     You aren't allowed to take photos in this sacred area.
     Was Julie allowed to leave work early?
     Tony won't be allowed to drive his father's car to the party tonight.