What to use - a/an or the?

The indefinite article a/an is used to talk about one particular person or thing when the listener does not know which one is meant.

• I saw a child in the toy store.
• She married an old man.
• A girl answered the phone.

The definite article is used when the listener knows which particular person or thing is meant.

• The child was crying. (You know which child - the one I met in the toy store.)
• I went to the doctor yesterday.
• The girl who answered the phone was polite. (You know which girl - the one who answered the phone.)

The first time you speak of something/somebody usea/an, the next time you repeat the same noun use the.

To talk about things in general, we normally use a singular countable noun with a/an.

• A spider has eight legs.
• An apple is red.

Note that we can also use a plural noun with no article to make a general statement.

• Children need love. 
• Spiders have eight legs. 
• Teachers must have patience. 
• Apples are red. 
• Computers are expensive. 

Sometimes we use a singular noun with the to make general statements. This is particularly common before the names of instruments and inventions.

• Who invented the computer? 
• I would like to learn the violin.