A, An, The: General or Specific

A, AN, THE Pictures:
The Rules of General or Specific Uses

Many students struggle with articles; trying to figure out when to use them, which one to use, and when they are not needed at all.  On the most basic level, THE should be used for SPECIFIC nouns, when your reader knows exactly which one you are referring to, or for nouns of which there is only one (the world). A should be used for GENERAL nouns, or when your reader does not know exactly which one you are referring to.

In the following pictures, you can see the difference between a specific noun, a general noun, and a noun of which there is only one (the universe). 

In the above picture, both brothers know which dog the writer is referring to, which means that the writer can use THE, instead of A.

In this situation, the reader does not know which dog the writer is referring to.  Therefore, the writer should use A, instead of THE.

In this situation, there is only ONE world, so the writer can use THE instead of A, and the reader will still know exactly which world he is talking about -- the ONLY world.