A singular countable noun always has an article or another determiner with it.
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, or when it does not matter which one.
- She married an old man.
- They have a big house in the city.
- You had better consult a doctor.
- I saw a hawker selling his wares on the street.
- A man came and knocked at the door.
A/an is also used to talk about any one member of a class.
- A teacher must have patience. (=any teacher)
- A spider has eight legs. (=any spider)
- A dog is faithful to its master.
- A parrot can repeat what you say.
Cases where the indefinite article should not be used
With plural and uncountable nouns
The indefinite article cannot used before plural and uncountable nouns.
- Apples are red.
- Computers are expensive.
- Time is money.
We cannot use the indefinite article with possessives. We use double possessives instead.
- He is a friend of mine. (NOT He is a my friend.)
With adjectives without nouns
We cannot use the indefinite article with an adjective alone (without a noun).
- She is a beautiful girl.
- She is beautiful. (NOT She is a beautiful.)