Here's why:
* Complete sentences need a subject and a verb. A prepositional phrase starts with a preposition (like "on," "in," "with," "under," etc.) and ends with a noun or pronoun, called the object of the preposition. It doesn't have a subject performing an action.
* Prepositional phrases modify other parts of a sentence. They act like adjectives or adverbs, adding information about nouns, verbs, or other phrases.
Example:
* Prepositional Phrase: "under the table"
* Sentence: The cat hid under the table.
In this example, "under the table" is a prepositional phrase that modifies the verb "hid." It tells us where the cat hid.
Let me know if you have other grammar questions!