This is the 13053rd most frequent Korean word.
동생은
"동생은" means "younger sibling is" or "as for the younger sibling" in English.
Here, '동생은' is used as the subject of the sentence, indicating that the younger sibling is the one who went to school.
동생은 학교에 갔어요.
(My) younger sibling went to school.
In this sentence, '동생은' serves as the topic, putting emphasis on the younger sibling's interest in soccer.
동생은 축구를 좋아합니다.
(My) younger sibling likes soccer.
Here, '동생은' introduces the individual whose birthday was celebrated, establishing the topic of the sentence.