The word "when" is not a preposition. It functions mainly as an adverb or conjunction, and it can also function as a noun. Prepositions are words that show the relationship between a noun or pronoun and other parts of a sentence, often indicating time, place, direction, cause, or manner, and they always start prepositional phrases that include an object (a noun or pronoun). Examples of prepositions are "in," "at," "on," "before," and "after," but "when" is used differently—it is not classified as a preposition. In summary, "when" is not a preposition but rather an adverb, conjunction, or noun depending on context.
