Here's why:
* Simple Subject: The main noun or pronoun that performs the action of the verb. It's the core of the sentence.
* Prepositional Phrase: A group of words that begins with a preposition (like "on," "in," "with," etc.) and ends with a noun or pronoun (the object of the preposition). It modifies another word in the sentence, adding information about location, time, etc.
Example:
* "The cat" is the simple subject in the sentence "The cat sat on the mat."
* "On the mat" is the prepositional phrase, modifying the verb "sat."
Since a prepositional phrase modifies another word, it can't contain the subject itself. The subject will always be a noun or pronoun standing independently, not within a prepositional phrase.