Learn Hardcore Korean: naneun chaekbangeseo changmuneul yeoreosseo.

는 is the topic particle, marking 나 (“I”) as the topic of the sentence. It often contrasts or emphasizes what comes after. 가 is the subject particle, used when introducing new information or focusing strictly on the subject. In this sentence, the speaker is setting “I” as the topic (“As for me, …”), so 나는 is natural.

Could you drop 나는 altogether?

Yes. Korean is a pro-drop language, so if the context makes it clear who the subject is, you can omit 나는 and simply say 책방에서 창문을 열었어. The listener will understand “I” did it.

Why is 에서 used in 책방에서 instead of 에?

에서 indicates the location where an action takes place. Here, opening a window happens at/in the bookstore, so you need 에서. In contrast, 에 would mark a static location or direction (e.g., 책방에 갔어 “I went to the bookstore”).

What’s the role of 을 in 창문을?

을 is the object marker attached to nouns ending in a consonant (here, 창문 ends in ㄴ). It flags “window” as the direct object of the verb 열다 (“to open”). If a noun ends in a vowel, you’d use 를 instead.

Why is the past tense 열었어 and not 열았어?

Korean attaches 았어 when the verb stem’s final vowel is ㅏ or ㅗ; otherwise it uses 었어 (or 였어 for 하다-verbs). The stem 열- has the vowel ㅕ, so it takes 었어, giving 열었어.

What speech level is 열었어, and how would you say it politely?

열었어 is casual (반말), used among close friends or younger people.
– Polite informal: 열었어요
– Formal polite: 열었습니다

Why does the verb come at the end of the sentence?

Korean follows Subject–Object–Verb (SOV) order. The verb always comes last. You can rearrange subject or object for emphasis (thanks to particles), but the verb stays at the end.

Is 책방 the same as 서점?

Yes—they both mean “bookstore.”
– 책방 is a pure Korean word, a bit more colloquial or warm.
– 서점 is Sino-Korean and slightly more formal. Both are widely understood.