单数的可数名词总是有冠词来修饰，例如a, an, the或者其他限定词

a, an

when we talk about something for the first time, or something that is part of a group or type.

We use a when the word that follows it begins with a consonant soud.
We use an when it's followed by a vowel sound.

I saw a good film yesterday.
Do you want a drink?
She has a university degree.
It took me an hour to get home.

the

when the listener already knows which thing we are talking about because it was mentioned before or because there's only one of them.

I'm going to take the dog for a walk.
Have you seen the car key?

no article

we normally use a plural or uncountable noun with no article

Birds eat worms
Children need a lot of sleep

examples

a book
a desk
an elephant
an hour(h不发音，所以参考后面的o)
a hospital
an egg
an umbrella
an actor
a doctor
a university(u的发音和You很像，所以用a)