This is the 8476th most frequent Korean word.
ㄴ씨는
"ㄴ씨는" is not a standalone Korean word but a phrase combining "ㄴ" (often a word-ending or particle) and "씨는" ("Mr./Ms. [name] is" or "as for Mr./Ms. [name]").
The word '박씨는' refers to Ms. Park, again using '씨' for honorific purposes and '는' to indicate the topic of the sentence.
박씨는 매우 친절해요.
Ms. Park is very kind.
Here, '이씨는' indicates Mr. Lee, employing '씨' as an honorific and '는' to make the person's capability in English the topic of the sentence.
이씨는 영어를 잘해요.
Mr. Lee is good at English.
The word '김씨는' refers to Mr. Kim, using '씨' as an honorific for addressing the person by their family name, followed by '는' as a topic marker.