Articles are words used before a noun and functions as an adjective. There are two types of articles – definite and indefinite. “The” is a definite article, while “a” and “an” are indefinite articles. Look at these examples:

1. A picture is worth a thousand words.
2. The man who came just now was my uncle.
3. An unusual picture is worth a thousand dollars.

In Example #1, the article “a” is used because we refer to any picture. Thus it is indefinite which picture we are referring to. However, in Example #2, we know which man we’re referring to - my uncle (the man who came to my house). Articles are used with singular nouns only; plural nouns do not have articles.

Zero article:

What? Another one? Yes. This is what I call an invisible article. When you write or say something factual, an article is not used. Here are some examples:

1. Dogs can’t fly.
2. Schools are closed on Sundays.
3. Computers need humans to provide input.

The rules for its correct usage is indefinite. Quite often, you’ll end up using what you feel sounds right.