Four words which anyone spending any extended time in Korea might encounter are Silla or Shilla, the dynasty which ruled most of the peninsula from 57 BC to 935 AD, also a classy hotel in Seoul; Jeolla-do, the province in the south-west of the peninsula, now divided into Jeollabuk-do or North Jeolla (including Jeonju) and Jeollanam-do or South Jeolla (surrounding but not including Gwangju); Hallasan, Mt Halla or Halla Mountain on Jeju-do, Korea’s highest mountain and Seollal, traditional new year, late Jan – mid-Feb.
These all have in common the spelling -ll-. You will almost certainly encounter these words in English transliterations before you see them in hangeul. Some acquaintance with hangeul might lead you to suspect that the spellings are 실라, 절라, 할라 and 설랄 respectively (compare 막걸리, mak-geol-li). But they’re not. They are 신라, 전라, 한라 and 설날.
The pronunciation of ㄴㄹ and ㄹㄴ as -ll- is another example of assimilation, where one of two consecutive sounds changes to become (more) like the other. This particular change happens sometimes in English, inherited from Latin. Illegal was originally in + legalis. Two differences: in English, the spelling changed to reflect the pronunciation (or maybe the spelling had already changed in Latin), and in English, it happens only some of the time: compare unlawful. As far as I know, the pronunciation changes all of the time in Korean, but the spelling doesn’t.
Something I didn’t know for some time (because guides to Korea don’t usually mention it) is that the names of the provinces are mostly built from the major cities in those provinces: Jeolla has Jeonju and Namju, Gyeongsang (south-east) has Gyeongju and Sangju, Chungcheong (central west) has Chungju and Cheongju and Gangwon (north-east) and Gangwon has Gangneung and Wonju. (Gyeongsang and Chungcheong are also now divided in two, and Gangwon is divided in two by the demilitarised zone.) Note that Namju and Sangju are relatively small cities. Compare England, in which Birmingham is now bigger than Warwick (the namer of Warwickshire) and Leeds is bigger than York (the name of Yorkshire).
Seollal fell a few months after the first time I moved to Korea. During an entry interview I asked a student what s/he had done for seol–nal, and s/he totally didn’t understand me until I wrote it on the board. But nal by itself means day, so seol–nal is simply (something-)day (I can’t quite find what seol by itself means). Maybe all those part of all those words have real meanings (eg, the 한 of 한라산 may be related to 하나, one – the number one mountain in Korea).
Slightly bucking the trend is 진로 soju, which is universally transliterated as Jinro, but pronounced jil-lo, even when written in English. I can’t remember that I’ve ever pronounced it, though; I either ask for soju and that’s the default brand, or take it from the fridge myself.