In many hanja words, initial 리을 and 니은 are dropped before a "y" /j/ or "i" /i/ sound, which correspond to the vowels ㅣ ㅕ ㅑ ㅖ ㅠ ㅛ. This often happens if the hanja constitutes the first syllable of a word (commonly a hanja-eo).
- 녀 女
- woman 여자 女子 has 녀 changed to 여
- 소녀시대 少女時代 remains in its original form
- 뇨 尿
- urea 요소 尿素 has 뇨 changed to 요
- diabetes 당뇨병 糖尿病 remains in its original form
- 리 理
- rationality 이성 理性 has 리 changed into 이
- reason 도리 道理 remains in its original form
- 련 練
- practice 연습 練習 has 련 changed to 연
- foolishness 미련 未練 remains in its original form
I've also checked my dictionary for 李, and found that it is actually 리, but since the Korean surname comes in the first syllable of a name, the surname is 이 instead (though I do have a Korean friend whose surname was anglicized as Rhee, not sure if it's a variation). However not every such case has the initial consonant dropped, for there are such surnames as 류.
For what I have observed this seems to be a rule for every such consonants at the start of the word (if at least the first syllable represents a hanja), before a /j/ or /i/, except for some surnames. Is this correct? Or are there more exceptions than just surnames? And how do the surname exceptions arise, perhaps a dialectal variation?
Add-on: if I want to address a person as 蓮 (the last word of her name), which is 련, but since I'm using only one syllable it becomes the first consonant of the word, should I use 연 instead?