Learn Hardcore Korean: hyeongwan yeope usaneul georeo dueosseoyo. - I hung the umbrella next to the entrance.

The construction -아/어 두다 attaches to a verb stem (here 걸다) to show that you perform an action in advance or for later convenience. In 걸어 두었어요, it implies “I hung it up (and left it) so that it stays there ready for later.”

Why use 걸어 두었어요 instead of just 걸었어요?

Saying 걸었어요 alone simply states “I hung (it).” Adding -어 두었어요 emphasizes that you hung it up and left it there intentionally—often to keep it in place for future use or to maintain order.

Why is the particle 에 used after 현관 옆?

The particle 에 marks a static location where something exists or an action takes place. 현관 옆에 means “next to the front door” as the location of the umbrella-hanging action.

Why do we say 우산을 걸다 here? Couldn’t we say 우산을 놓다 instead?

걸다 means “to hang,” appropriate when you put something on a hook or hanger.
놓다 means “to put” or “to place” on a surface.
Since an umbrella is being hung (on a rack, hook, or stand), 걸다 is the correct verb.

What nuance does the past tense -었어요 add in 걸어 두었어요?

Using -었어요 indicates the action was completed in the past. It conveys that at some point earlier, you hung the umbrella up and it remains hanging now.

Could I say 현관 옆에 우산을 놓아 두었어요 instead?

Yes, but it changes the image: 놓아 두었어요 suggests you placed the umbrella on a surface (e.g., on a bench or floor). If your umbrella is hanging, 걸어 두었어요 is more natural. If you left it resting somewhere, 놓아 두었어요 works.