Learn Hardcore Korean: ibeon hakgieneun gwamogi manhaseo bappayo. - I’m busy this semester because I have many subjects.

에 marks the time frame (“during/at this semester”), and 는 marks the topic. Together as 에는, it gives the nuance “as for this semester,” highlighting that what follows is about this specific term.

Why is 과목 marked with 이 (subject marker) instead of 은?

Because 많다 is a descriptive verb that takes 이/가 to mark its subject (“subjects are many”). If you used 은, it would change to a topic marker and give a different nuance—often sounding unnatural in this causal structure.

What does the connective ending -아서 in 많아서 indicate?

The ending -아서/어서 links two clauses with a causal meaning: it literally means “because…,” so 과목이 많아서 = “because there are many courses…”

Can we use -고 instead of -아서 to link the clauses? If not, why?

No. -고 simply lists two statements without implying cause (“and…”). To express “because,” you need -아서/어서 (or alternatives like -기 때문에).

Why doesn’t the sentence include 저는 (“I”)?

Korean often omits the subject when it’s clear from context. Here 저는 (“as for me”) is understood, so it’s dropped for naturalness.

Why is 바빠요 used instead of 바쁘다, and what level of politeness does it convey?

바쁘다 is the dictionary (infinitive) form. Conjugating with -요 yields 바빠요, the present-polite style (해요체), appropriate for polite everyday conversation.

Could we replace 과목이 많아서 with 과목이 많기 때문에? How does that change tone or formality?

Yes. 과목이 많기 때문에 바빠요 is perfectly correct and slightly more formal or written. -아서/어서 is more conversational and common in spoken Korean.

Is 많다 a verb or an adjective in Korean, and how does it conjugate?

In Korean grammar, 많다 is a descriptive verb (형용사), similar to an adjective in English. It conjugates like other verbs: 많다 → 많아서 (reason), 많아요 (polite present), 많았다 (past), etc.