Learn Hardcore Korean: chilpan yeope sigyega geollyeo isseoyo. - There’s a clock hanging beside the blackboard.

걸려 있어요 literally means “is hung” or “is hanging.” It’s the descriptive/state form of the verb 걸다 (to hang) expressed as 걸리다 (to get hung) plus -어 있다 (to remain in that state). So you’re saying “the clock is in a hung-up state.”

Why is 걸려 있어요 used instead of just 걸어요?

걸어요 (active) means “I hang (something).”
걸려 있어요 (passive/state) describes the resulting state: “(Something) is hanging.”
Since the sentence tells us where the clock already is (hanging) rather than someone hanging it right now, we use 걸려 있다.

What role does 에 play in 칠판 옆에?

The particle 에 marks a location with verbs of existence or placement. Here 칠판 옆에 means “at/to the side of the blackboard.” Whenever you describe where something 있다, 걸려 있다, 앉아 있다, etc., you attach 에 to the location noun.

Why is 시계가 used (with the subject marker) instead of 시계를 (object marker)?