In traditional grammar, a simple subject is the particular noun or pronoun that tells who or what a sentence or clause is about. In traditional grammar, a simple subject is the particular noun or pronoun that tells who or what a sentence or clause is about. A simple subject may be a single word (e.g., Christmas is coming), a multi-word proper noun (Santa Claus is coming), or the key noun or pronoun in a complete subject (The zombies in the basement are coming upstairs).