We use a when we talk about something for the first time
Example
I saw a good film yesterday
Do you want a drink
We use a when the word after it starts with a consonant sound and we use an when the word starts with a vowel sound.
Example
She has a university degree
It took me an hour to get home
We use the when the listener knows which thing we are talking about, for example because it was mentioned before.
Example
I’m going to take the dog for a walk
Have you seen the car key?
When we talk about general things (for example in plural forms or uncountable nouns), we don’t use any articles
Example
Birds eat works
Water freezes at 0 degrees
Children need a lot of sleep