Expert School

A, An or The?

When do we say "the dog" and when do we say "a dog"? (Today we talk only about singular, countable nouns.)
The and a/an are called "articles". We divide them into "definite" and "indefinite" like this:

Definite: The
Indefinite: A/An

We use "definite" to mean sure, certain. "Definite" is particular.
We use "indefinite" to mean not sure, not certain. "Indefinite" is general.
When we are talking about one thing in particular, we use the. When we are talking about one thing in general, we use a or an.
Think of the sky at night. In the sky we see 1 moon and millions of stars. So normally we would say:

I saw the moon last night.
I saw a star last night.

Of course, often we can use the or a/an for the same word. It depends on the situation, not the word. Look at these examples:
We want to buy an umbrella. (Any umbrella, not a particular umbrella.)
Where is the umbrella? (We already have an umbrella. We are looking for our umbrella, a particular umbrella.)

Teacher Gustavo