Simple Definition:

A subject in a sentence is the noun [a person, place, thing, or idea] or pronoun [a word used instead of a noun, such as “it” or “they”] that performs the action or is being described by the verb.

Examples:

1. Maria read a book. (The subject is “Maria.”)
2. They are playing soccer. (The subject is “They.”)
3. The cat is sleeping. (The subject is “The cat.”)

Very Simple Definition:

A subject in a sentence is the person, thing, or animal doing something or being talked about.

Examples:

1. Maria read a book. (Maria is doing something.)
2. They are playing soccer. (They are doing something.)
3. The cat is sleeping. (The cat is being talked about.)