@Yacchan ?? "I'm a quick study" is literally proper English lmao. It's an expression,and one that's particularly common in formal speech, hence why the translator used it.
@Yacchan google could've done it for you but here is a link to the merrium Webster dictionary showing that "quick study" is a noun, just like it's used in this chapter.
@Yacchan you can use more formal words or informal ones. For example: to request something or to ask for something means basically the same but the first one is the formal one here. Formal speech indicates higher level of education, having a highe status or talking to someone who has a higher status than you in society or whatever. The top in the story is a Lord which is the reason the translator chooses mostly formal speech.