Hello, fellas. This session is still related to the previous session about the use of an article.
Beside a/an, the other article is the. The precedes
1) singular count nouns
e.g. the cat
2) plural count nouns
e.g. the cats
3) noncount nouns
e.g. the fruit
The is used when the speaker and his/her listener are thinking about the same specific person(s) or thing(s).
e.g. Have you fed the cat?
In the example above, the speaker and the listener are thinking about the same specific cat. The listener knows which cat the speaker is talking about. There is only one cat about which the speaker could be talking.
The is also used when the speaker mentions a noun the second time.
e.g. I had a book. I gave the book to Sally.
I had a book constitutes the first mention. In the second mention, the listener knows which book the speaker is talking about: the book the speaker had.
Betty Schrampfer Azar, Fundamentals of English Grammar: Third Edition