Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They often describe location, time, or direction.
* "When" is a conjunction, a word that joins words, phrases, or clauses. It indicates time.
Let's look at the other words:
* Behind: Shows location (e.g., "The cat is hiding behind the couch.")
* Around: Shows location or direction (e.g., "Walk around the block." or "The sun shines all around.")
* Into: Shows direction (e.g., "He walked into the room.")
* Toward: Shows direction (e.g., "She walked toward the door.")