•We use a to talk about an indefinite or general noun.
Example: Can I have a cookie? (I would like any cookie, it doesn’t matter which one)
•We use an to talk about an indefinite or general noun with a vowel sound.
Example: Do you have an umbrella? (any umbrella, it doesn’t matter which one)
•We use a and an to mean “one”
Example: Can I borrow a dollar? (I just need one dollar)