Here's why:
* A preposition is a word that shows the relationship between a noun or pronoun and another word in the sentence. It typically comes before a noun or pronoun (e.g., "in the box," "on the table").
* A complete sentence needs a subject and a verb. It expresses a complete thought.
For example:
* Incorrect: "On the table." (This is just a prepositional phrase, not a sentence.)
* Correct: "The book is on the table." (This has a subject, "book," and a verb, "is.")
Let me know if you have any other questions!