이/가' is used as subject marker , hence indicating the syntactic role (e.g. whether the noun is a subject, an object, indirect object etc.) of the noun preceding it as the subject . eg. 이것이 연필이에요=This (subject) is a pencil whereas '-은/는' is topic marker, which function is not to indicate syntactic roles of the noun but rather marking the noun as the topic of conversation/statement, emphasizing the singularity of the noun.  eg. 이것은 연필이에요.=IT’S a pencil (emphasis on 이것)  이것이 (연필)이에요. This is A PENCIL. (이것)은 연필이에요. THIS is a pencil. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 한국말이 (재미있어요). Korean is INTERESTING. (한국말)은 재미있어요. KOREAN is interesting.