증인은

the witness

→ 증인
phrase Rare #29,018
the witness 은 marks 증인 as the topic of the sentence and can add a contrastive sense, like “as for the witness.”
KO: 증인은 법정에서 진실을 말했습니다.
EN: The witness told the truth in court.

증인은?

What about the witness?

— 어 퓨 굿 맨 (1992)