I said "this heavy door is difficult to close" and it got marked wrong. :-(
I think you're supposed to use the gerund, as that's the name of the skill.
Report it then. But I believe the subject of this sentence is the ACTION of CLOSING and not the door itself, that's why it's wrong
I put 'Shutting this heavy door is difficult' and got it wrong. When talking about doors in English, closing and shutting are interchangeable and does not change the meaning in the slightest.
Still don't understand 것이/을/은. My understanding is you add it when the object is marked as a subject?