a and an (indefinite article)

Use a / an with singular countable nouns when talking about something for the first time or when it could be any one of its kind:

ArticleWhenExample
aBefore consonant soundsa dog · a book · a university
anBefore vowel sounds (a, e, i, o, u)an apple · an egg · an orange · an hour
Sound, not spelling: Use an before a VOWEL SOUND: "an hour" (the h is silent → sounds like "our"), "an honest man". Use a before a CONSONANT SOUND: "a university" (sounds like "yoo-niversity" → consonant y). It's the sound that matters, not the letter.

the (definite article)

Use the when both speaker and listener know which specific thing you mean:

When to use theExample
Second mention (already introduced)I have a dog. The dog is black.
Only one existsthe sun · the moon · the internet
Both know which oneCan you close the door, please? (the door of this room)
SuperlativesShe is the best student in the class.

No article (zero article)

Some nouns take no article at all:

WhenExample
Plural nouns in generalI like cats. (cats in general)
Uncountable nouns in generalI drink water. (not "a water")
Names of peopleMaria is my friend. (not "the Maria")
go to school / work / bed / homeShe goes to school every day.
Names of languages, sports, mealsI speak English. We play tennis. I eat breakfast.
First mention = a/an, second mention = the: "I saw a man in the street. The man was wearing a red hat. The hat was very big." — first time we mention the man: a man. After that we know which man: the man.

Exercise

1. I have ______ orange in my bag.

2. She is ______ best teacher in the school.

3. I saw ______ film last night. ______ film was very funny. (first blank)

4. ______ sun is very hot today.

5. She goes to ______ school by bus. (everyday routine, no article)