Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They often indicate location, time, or direction.
* Adverbs modify verbs, adjectives, or other adverbs.
* Adjectives modify nouns or pronouns.
Example:
* For dinner, we had pizza. ("For" shows the relationship between "dinner" and "we had pizza")
* We waited for hours. ("For" shows the relationship between "waited" and "hours")
Let me know if you have any other grammar questions!