Simple Definition:
A subject in grammar is the main noun or pronoun that performs the action in a sentence [a group of words that express a complete thought]. The subject tells us who or what the sentence is about.
Examples:
- Sarah is walking her dog.
- They are playing soccer in the park.
Very Simple Definition:
The subject is the person, thing, or animal doing the action in a sentence.
Examples:
- The dog is barking.
- John is eating pizza.