Learn Hardcore Korean: doseogwaneseo sueobeul deureoyo.

에서 marks the place where an action happens (“at/in the library”). If you said 도서관에, that would usually indicate a destination (“to the library”), not the location of the action. Since you’re taking a class at the library, you need 도서관에서.

What does 수업을 들어요 literally mean? Why use 들어요 with 수업?

The verb 듣다 literally means “to hear” or “to listen.” However, in the fixed expression 수업을 듣다, it means “to take a class” or “to attend a lecture.” So 수업을 들어요 = “I take/attend a class.”

Why is 수업 marked with 을 instead of 은 or 이?

을 is the object particle, marking 수업 as the direct object of the verb 듣다. You need an object marker (을/를) because you’re “taking” (listening to) something, namely the class (수업을 듣다).

Can I change the word order? For example, 수업을 도서관에서 들어요?

Yes. Korean word order is relatively flexible because particles show each noun’s role. Both
• 도서관에서 수업을 들어요
• 수업을 도서관에서 들어요
are correct and mean the same thing. Putting the topic or emphasis first can slightly shift the focus, but the basic meaning stays.