This is the 13731st most frequent Korean word.
마르코스는
"마르코스는" translates to "Marcos is" or "Marcos" (as a topic, in Korean grammar).
In this sentence, '마르코스는' marks Marcos as the subject of the sentence, indicating who went to school.
마르코스는 학교에 갔습니다.
Marcos went to school.
Here, '마르코스는' is used to introduce Marcos as the noun performing the reading action.
마르코스는 책을 읽고 있어요.
Marcos is reading a book.
In this example, '마르코스는' sets the subject as Marcos, describing him as a kind person.