는

Marks the topic, after a vowel

Lifts a word to the front as the thing the sentence is about — "as for X". It is what speakers use to open a segment ("today's first topic is...", "last week was...") and to set one thing against another, so it often carries a quiet contrast with whatever was not mentioned. It is used after a vowel; 은 is the same marker after a consonant. Against 이/가, which points at the specific doer, this one sets the frame and the rest of the sentence fills it in.