One of the hardest part or perhaps the most confusing of learning Korean is the pronunciation. They have a lot of phonological rules that are hard to remember when pronouncing it so my goal is to simplify matters. I’m starting with this one because it’s special in its own right since it deviates from the patterns of the others.
In class or in whatever textbook they tell you that an ㄴ before ㄹ can become two ㄴ (also if it’s backwards) but that also that this sequence of sounds can become twoㄹ instead. Usually (more like never), they don’t clarify when it becomes exactly ㄴ or ㄹ. Apparently, the assimilation of one to the other relies entirely on whether it’s Sino-Korean word or Pure Korean and whether there’s a morpheme boundary or a word boundary.
The rules of nasalization and lateralization of these two phonemes is bound by their origin. The reason this happens it’s because Korean doesn’t accept the successive combination of ㄴ and ㄹ.
1. If the word is Pure Korean, it will always become ㄹㄹ.
In order to know that, you must know about the vocabulary. If the word starts with ㄹ or ㄴ, it’s a Sino-Korean word because Pure Korean words doesn’t have words that start with those. However, there are words who suffered even more change to accommodate the Korean phonological rules. There’s for example 년 (年) which means year, it’s a Sino-Korean word but when it starts the word, it becomes 연 like in 연세 (年歲) which means age.
Note: This rule doesn’t include loanwords from English.
2. In the case it’s a Sino-Korean word, it can become either ㄴ or ㄹ.
A. If there’s a word boundary, then ㄹ becomes ㄴ.
This is the hardest part for those who doesn’t know a lick of Hanja, because they don’t distinguish word boundary by space but by meaning in itself (if there’s two independent words together).
B. If there’s a morpheme boundary, then ㄴ becomes ㄹ.
It is a bit easier to know the morpheme boundary because, if both form a word that holds a meaning by itself then, the ㄴ becomes an ㄹ. A clear example of it is 신라 (新羅) of Silla Kingdom. The meaning more or less of the first character stands for 새 (new) so it is like a counter (which counts as a morpheme but is an adverb and not a free form) and the second one stands for 그물 (net) which it is a word in itself. Therefore, the ㄴ becomes an ㄹ.
Honestly speaking, it doesn’t make much difference for anyone who doesn’t know Hanja. However, there is a method to this madness (lol) because at least when you learn Korean, you usually know which words are Sino-Korean and which ones Pure ones so in the end you can get away with changing the combination to ㄹ.