This is the 17202nd most frequent Korean word.
서씨는
"서씨는" means "Mr./Ms. Seo" or "Seo family" in the topic marker context.
Here, '서씨는' is used to introduce a person's name and to indicate that Mr. Seo is the subject of the following statement.
서씨는 음악가입니다.
Mr. Seo is a musician.
In this sentence, '서씨는' denotes Mr. Seo's involvement in today's activity, specifying the subject of the action.
서씨는 오늘 공연에 참여합니다.
Mr. Seo is participating in the performance today.
This usage of '서씨는' implies that Mr. Seo has diverse activities, with the phrase indicating who is performing the actions described.