Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They typically indicate location, time, or direction. Examples: *on*, *in*, *at*, *to*, *from*.
* Adverbs modify verbs, adjectives, or other adverbs. They provide more information about how, when, where, or to what extent something happens. "Again" tells us that something is happening repeatedly or a second time.
Example:
* Preposition: "The book is on the table." (On tells us the location of the book)
* Adverb: "I read the book again." (Again tells us when the action of reading happened)