Used when we (speaker and listener) don’t know which one, or it doesn’t matter which one
- I need a cup; could you get me one, please? (It doesn’t matter which one, I just need a cup.)
- She wants a doll for her birthday.
Used to talk about any one member of a class.
- A car is a necessity for many people. (any car)
- An elephant is big and gray.
Used to classify people or things.
- She’s a waitress.
- “What’s that?” “Oh, it’s just a wind chime, that’s all.”
Plural and uncountable nouns generally do not get an article—use some or any.
- I saw some geese flying overhead.
- Water, milk, bread, sugar, etc.
- Please give me some juice.
- Do you have (any) money?
- The store was out of butter.
When “a/an” is necessary:
- Negative expressions
- “Let me borrow your calculator.” “I don’t have a calculator.” (Not I don’t have calculator.)
- Prepositions
- He sat down in the shade of a tree. (Not … of tree.)
- Fractions
- two-thirds of a mile (Not two-thirds of mile)
When “a/an” is NOT used:
- With adjectives alone (without a noun).
- It’s a nice day. (The article is used, since there is a noun.)
- It’s nice. (Not It’s a nice.)
Possessives—use the phrase a _______ of mine/yours/ etc.
- She’s my teacher.
- She’s a teacher of mine.