은 after a consonant (책은) · 는 after a vowel (저는)
topic marker (as for …); contrast
Example
저는 학생이에요.
I am a student.
Word parts
은 after a consonant (책은), 는 after a vowel (저는 = as for me).
Good to know
은/는 marks what you're talking about; 이/가 marks new information. 'Who came?' → 누가 왔어요?
Related words