A and an

Use a or an before one non-specific singular count noun. A car means one car, but not a particular car already known to the listener.

Choose the article by sound, not spelling. Use a before a consonant sound: a book, a teacher, a university. Use an before a vowel sound: an apple, an old bike, an English student.

A singular count noun normally needs a determiner. Say I have a car, not I have car. The determiner can also be the, my, this, or another word that identifies the noun.

Use a or an when you first introduce something: There is a cat in the garden. Later, when both speakers know which cat you mean, use the: The cat is black.

The article does not change for grammatical gender. It only signals one non-specific countable person or thing and helps the noun sound complete.