은/는

은 / 는 — topic particle

What you are talking about — contrast, scene-setting, and “as for…”.

As for X…

은/는 names the frame. 오늘은 바빠요 “As for today, I’m busy” (maybe other days are fine). Dropping it is common in speech when the topic is obvious.

Do not translate it as English “the.” It is closer to putting a header on the sentence.

저는 미국 사람이에요.

jeoneun miguk saramieyo.

I’m American.

커피는 안 마셔요.

keopineun an masyeoyo.

I don’t drink coffee (though I might drink other things).