Here's why:
* Prepositions show relationships: Prepositions like "in," "on," "to," "from," etc., describe the relationship between a noun or pronoun (the object of the preposition) and another word in the sentence.
* Subject sentences are complete thoughts: A subject sentence must have a subject (who or what the sentence is about) and a verb (what the subject is doing).
* Objects of prepositions are not complete thoughts: They are single words or phrases that function as nouns or pronouns, and they don't express a complete action or idea.
Example:
* Incorrect: "I went to the store where they sell the best cookies."
* The phrase "where they sell the best cookies" is a complete sentence, and it's functioning as the object of the preposition "to." This is grammatically incorrect.
* Correct: "I went to the store that sells the best cookies."
* "That sells the best cookies" is a relative clause modifying the noun "store." It is not the object of the preposition "to."
In summary: The object of a preposition can be a noun, pronoun, or a noun phrase, but it cannot be a complete sentence.