I’m having trouble understanding how classes relate to their methods. Is a method something that the object does, or something that’s done to it? Or is this a different concept entirely?
Specifically, in a library’s software system, should the borrow() method belong to the class representing the library patron, or the class representing the item that the patron is borrowing? My intuition is that it should read like patron.borrow(copy), like English sentence structure, subject.verb(object); but my instructor says that’s Wrong, and I don’t understand why he would have borrow() belong to the Copy class (and he doesn’t really explain things too well). I’m not looking for justification, but can someone just explain the proper relationship?
Edit: This question was closed as “off topic”. I don’t understand. Are software design questions not appropriate for this site?
Some generic words first.
Software construction is not something which should be governed by English language rules or "beauty" or whatever, it's engineering discipline. Think of whether your design solves the problem, whether it will be maintainable, whether it will be testable, whether it will be possible to parallelize development and so on. If you want something more formalized take a look at the "On the Criteria To Be Used in Decomposing Systems into Modules" by D. L. Parnas.
As for your library example. Imagine you have a Copy outside of library, shoult it have borrow method then? How the borrowing is registered? Are you ok with either Copy or Patron classes responsible for data storage? It looks more appropriate to put borrow into a Library class. Responsibilities will be clearly divided, you wouldn't need to know much about borrowing to implement Copy and Patron and you wouldn't need much details about them to implement Library.