Learn Hardcore Korean: gonghangeseo hwanjeonhaesseoyo.

에서 indicates the place where an action takes place (“at the airport”). 에 alone marks a static location (“to the airport” or “in the airport”) or a point of existence, but 에서 emphasizes “here is where I did something.”

Why isn’t there a direct object particle like 돈을 before 환전했어요?

환전하다 (“to exchange currency”) already includes the object idea, and Korean often omits objects when they’re clear from context. You could say 공항에서 돈을 환전했어요, but dropping 돈을 sounds more natural once it’s understood you exchanged money.

What is the meaning and origin of 환전?

환전 (換錢) is a Sino-Korean noun meaning “currency exchange” (literally “change money”). With 하다 it becomes the verb “to exchange money.”

Why is the verb in the form 했어요 instead of 했다 or 했어?

했어요 is polite informal speech, used in everyday conversation with people you’re not extremely close to.

했다 is plain past, more common in writing or very casual talk.
했어 is informal, used among close friends or younger people.