"Advanced spells are very complicated. To be able to do one, you have to be able to fit it into your head, but everybody at some point runs into a limit on how well they can do that with standardized notation. People use different ways of condensing and marking all the concepts they need to juggle in a way that meshes better with how they think, and no two wizards write a complicated spell the same way. Sometimes there's even a few ways to do the same one in the same notation - big paper is most efficient in terms of how long it takes to write down, but there's ways to do it paginated in books, or all lengthwise in a more standard scroll. Once it's written down, there's magic that lets you understand somebody else's notation, there's a spell for it which is simple enough to learn the standard notation way, but if you want to get the spell down permanently as opposed to just cast it once you want to write it down your own way and study that. So I had Fëanáro write down his, stare at it so I could see it through his eyes and copy it just how he had it, cast the spell so I could understand it a bunch of times since then because I can't cast the spell through him so I need my own scroll of it, and copy it into my own."