Use and Form
We use a or an in front of singular, countable nouns. We use an in front of nouns that start with a vowel and a in front of nouns that start with a consonant:
A book / a window
An actress / an exam
We use a when it doesn’t matter which person or thing we are talking about, or when we talk about the person or thing for the first time:
We need a car.
They have a big garden. (I’m talking about it for the first time)
We use the when we talk about a person or thing again and the other person knows who or what it is:
They have a cat and a dog. The dog is dangerous.
Feed the dog. She is hungry. (our dog)
There are a lot of phrases without articles:
At night, at home, at school, in hospital, in bed, by train, by plane, by taxi, before lunch, after dinner, go to school, go to work, go home, have breakfast, have lunch.
Minvokavit 