Articles can either be definite or indefinite. They combine to a noun to indicate the type of reference being made by the noun.

Definite article: The

Indefinite article: A / An

Used when we don’t specify the people or things we are talking about.

Examples: I met an online friend.

I work in a factory in New York.

I borrowed a pencil from a passenger sitting next to me.

Used when talking about a specific person, topic or things that both the speaker and the listener know.

Examples: The football match last night was intense.

The news is filled with the new election.

Other than that, when we speak about someone or something for the first time, we use the indefinite articles a and an. The next time the same person or thing is repeated, we use the definite article the.

Examples: I live in a condominium. The condominium is in the heart of the city.

We can also opt to not use articles when:

1.   Talking about countries, states, counties or provinces, lakes and mountains except when the country is a collection of states such as "The United States".

Examples: He lives in Kuala Lumpur near KLCC .

They live in Northern Malaysia.

They climbed Mount Everest.

2.    With plurals and uncountable nouns to talk about things in general:

Examples: He writes songs.

She likes cakes.

Do you like jazz music?

She ate bread with butter in the morning.

The can be used with uncountable nouns, or the article can be dropped entirely as mentioned above.

1. "The two countries reached the peace after a long disastrous war" (some specific peace treaty) or "The two countries reached peace after a long disastrous war" (any peace).
2. "He drank the water" (some specific water- for example, the water his wife brought him from the kitchen) or "He drank water." (any water)

It is unusual to use a/an for uncountable nouns. You can't say "I’d like a milk” (x)

a/an can be used only with countable nouns.

1. I'd like a piece of cake.
2. I lent him a book.
3. I drank a cup of tea.