Why you use 'the' in a sentence?

Answer 1

The is called the definite article, other articles are a and an. They come before nouns.

The means we know about the noun the is referring to. In the sentences:

The dogs is sick - we know about this dog, we know what dog is sick.

A dog is sick - we don't know about this dog, this is any dog.

So we use the to refer to a noun that the speaker and listener or writer and reader already know about.

I saw a cat on the roof. The cat was big and black.

In these sentences we use a cat first because we don't know anything about the cat. Then in the next sentence we use the cat because we now know the cat we are talking about, it is the cat on the roof.