The main part of a sentence that tells us who or what the sentence is about is the subject. The subject is the noun or pronoun that performs the action or is being described in the sentence. It is the main focus of the sentence and provides information about who or what the sentence is talking about.

Explanation:

1. Subject as the main focus:
The subject is the main focus of the sentence. It provides the central idea of the sentence and is usually located at the beginning of the sentence. It tells us who or what is being talked about.

Example: "The cat is sleeping."
In this sentence, "The cat" is the subject, and it tells us that the sentence is about a cat.

2. Subject as the performer of the action:
The subject is the noun or pronoun that performs the action in the sentence. It is the doer of the verb and is responsible for the action taking place.

Example: "John is eating an apple."
In this sentence, "John" is the subject, and it tells us that John is the one performing the action of eating.

3. Subject as the one being described:
The subject can also be the noun or pronoun that is being described or talked about in the sentence.

Example: "The book is interesting."
In this sentence, "The book" is the subject, and it tells us that the sentence is describing the book as interesting.

Conclusion:
The subject is the main part of a sentence that tells us who or what the sentence is about. It can be the performer of the action or the one being described. It provides the central idea and focus of the sentence.