Learn Hardcore Korean: sueobi kkeutnago naseo doseogwane gayo.

-고 alone can link two actions (“…and…” or “…then…”), but adding 나서 (from the verb 나다, “to occur”) forms -고 나서, which specifically means “after finishing (the first action), then…”. It emphasizes that the first action is fully completed before the next one starts.

What role does 이 play in 수업이 끝나고?

이 is the subject particle attached to 수업 (“class”). Because 수업 ends in a consonant (ㅂ), you use 이 to mark it as the subject of the verb 끝나다 (“to end”).

Why is 도서관에 used instead of 도서관에서 or 도서관으로?

With 가다 (“to go”), 에 marks the destination (“to the library”).

도서관에서 가요 would mean “go from the library” (starting point).
도서관으로 가요 also means “go toward the library,” but 에 is the neutral, most common way to express simply “going to” a place.

What level of politeness is 가요?

가요 is the polite informal present tense of 가다.

Use 가요 for everyday polite conversation (with classmates, co-workers, people you’re not super close to).
For formal speech you’d say 갑니다; for casual/friendly you’d say 가.

Could I say 수업이 끝나고 도서관에 가요 (without 나서)?

Yes. 수업이 끝나고 도서관에 가요 is perfectly natural. Omitting 나서 makes it a bit more casual and less focused on “completion,” but the overall meaning (“After class ends, I go to the library”) stays the same.

Is it possible to use 뒤에 instead of -고 나서?

Absolutely. You can say 수업이 끝난 뒤에 도서관에 가요. Here, 끝난 is the past adjective form of 끝나다, and 뒤에 means “after.” It conveys the same sequence of events.

Why is 수업이 끝나고 나서 placed before 도서관에 가요?

Korean typically follows Subject-Object-Verb order and places time or condition clauses before the main verb. So sequence phrases like 수업이 끝나고 나서 naturally come at the beginning, setting when the action happens.

Why does 끝나고 often sound like 끈나고 in speech?

Korean consonant assimilation causes ㄷ in 끝 to change to ㄴ before another ㄴ (as in 나고). So 끝나고 is pronounced more like [끈나고] in fluent speech.