What you on about, Andreas? Lol 더 and 던 are different things 던 is a complicated one that honestly I don't even understand that well, but it means like "used to". Even Korean people mess it up sometimes. 던.. It's when you're recalling something. Something you used to do. Or something you did in the past, that you didn't finish. Something that was, that isn't so at the current time. 내가 읽던 책 the book that I was reading (at that time, now I'm not reading it or haven't finished the whole book) 읽은 책 would mean you finished all of the book 이뻤던 여자 the girl that was pretty (but isn't anymore) 춥던 날씨 the weather that had been cold I think that used 추운 날씨 would be awkward, but because the weather is not in fact cold anymore. Person above conjugated 춥다 incorrectly btw

http://talktomeinkorean.com/lessons/l8l4/ Talk to me in Korean explains this very well I think. I know it is the same as Emily posted (more or less at least), but it is nice to get confirmation from a Korean source. As I don't use textbooks to learn, I now and then search that site for grammar that puzzles me. :)

Also it is supposed to be "became warm suddenly", but I guess you just mistyped.

I'm not sure about that, but adjective + 지다 is "become". I guess the ㄴ indicates adjective. If it were 추운 I would understand it, as the adjective for "cold". But 더 I'm not sure of, other than it isolated means "more".

The expression is trying to convey a change in the weather, however, it is a terrible example. Staying with the theme, maybe it could be changed to: The weather, which had been cold, suddenly become freezing. ( the use of a stronger adjective gives the sentence a better balance )