Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. In this case, "behind" shows the relationship between someone or something and a state of progress or completion.
* Prepositions often answer questions like "where?" or "when?" For example, "He is behind in his work" implies a location or state of being "behind."
Examples:
* "She is behind in her payments."
* "The team is behind in the game."
Let me know if you have any more grammar questions!