Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. They usually indicate location, time, or direction. Some common prepositions are: on, in, at, to, from, by, with, etc.
"Twice" is an adverb. It modifies verbs, adjectives, or other adverbs, telling us how many times something happens.
For example:
* I ate the cake twice. (twice modifies the verb "ate")
* She is twice as old as him. (twice modifies the adjective "old")
Let me know if you have any other questions about word types!