Here's why:
* Subjects perform the action: The subject of a sentence is the noun or pronoun that performs the verb's action.
* Prepositional phrases modify: Prepositional phrases provide additional information about nouns or verbs in a sentence. They don't act as the main subject.
Example:
* Incorrect: "Under the bed" is a scary place.
* Correct: "The space under the bed" is a scary place.
In the incorrect example, "under the bed" is a prepositional phrase. It modifies the noun "place." The correct sentence uses the noun phrase "The space under the bed" as the subject, performing the action of being "scary."
Key takeaway: A prepositional phrase can never be the subject of a sentence because it doesn't have the grammatical function to perform the verb's action.