Articles - a, an, the

English has the indefinite article “a”, “an”, the definite article “the” and the so called zero article, when the other two are not used.

Indefinite Article

1. The indefinite article can be seen as meaning "one", which is actually did in the past:
I saw a woman there.
He is a student.
Rome was not built in a day.

2. It is used in front of every countable singular noun, except when there is a definite article (the), a pronoun (my, his; this, that, etc.) or a number (one, five, etc.).
I bought a book / a cake / a table.
BUT
I bought water. (uncountable, you cannot say 'one water')
I bought books. (plural)
I bought the table. (there is 'the')
*If there are adjectives in front of the noun, the indefinite articles goes in front of them all:
He is a tall handsome man.
I have a beautiful small leather purse.

3. Difference between „a” and „an”:
“a” is used before a consonant sound: a car, a bed; a university, a walk (they start respectively with /w/ and /j/, which in English are consonants); a horse, a wrist /rist/.
“an” is used before a vowel sound: an apple, an orange, an undertaker; an hour, an honour (“h” is silent, which means that the word starts with a vowel sound).

Definite Article

1. We use it when it is clear what we are talking about. This could be because
1.1. there is only one:

Have you met the new teacher? (there might be many teachers but the new one is only one)


The moon is bright tonight.
For the same reason it is used with superlatives:
This is the best performance I have ever seen!
He is the tallest man on the planet.
1.2. we are referring to something that has already been mentioned or we are specifying which one as we speak:
Did you call the journalist who wanted to interview you?
Take a look at the boy over there next to the door.

2. With names
2.1. geographical names in the plural: the Himalayas, the Azores, the Netherlands
2.2. countries that contain „State, Republic, Emirate, Kingdom”: the USA, the Czech Republic, the UAE, the UK
2.3. names of rivers, oceans and seas: the Danube, the Thames, The Black Sea, the Mediterranean.

2.4. newspapers: The Times, The Wall Street Journal


Do NOT use the definite article when there is a possessive pronoun or 's for possession:
That is the house.BUT
That is Peter’s house.
That is his house.

Zero article

That simply means that there is neither a/an nor the, for example before uncountable nouns (I like orange juice.).

Related articles:
Uncountable nouns in English
When can you use "a" or "an" before an uncountable noun?

Exercises:
Definite, Indefinite and Zero Article
Definite, Indefinite and Zero Article (2)
Articles With Geographical Names
Other exercises:
Articles