@duolingo: if the form "to + verb" is accepted here in this exercice of the section "Gerund" , shouldn't it be accepted in the other sentences/exercices of this same section as well? It is mainly considered as wrong to use the form to + verb instead of verb+ing... An explanation would be appreciated to avoid confusion for those who learn korean, because for the moment it implies that these two (english) forms translates differently in the whole section,exept here!!!
The main problem is the lack of context.<h1>English</h1>
A plain statement like the one given can be interpreted in 2 ways, either as a statement describing a specific instance (gerund):
It is difficult coming here (actual)
or a stated generality (infinitive):
It is difficult to come here (general situation)
Both translations should therefore be recognized as correct.
However, sometimes the choice of gerund vs infinitive can be determined by the tense of the final verb.
The gerund as said is used to describe an actual and specific event i.e. one that has happened, hence mostly PAST.
여기 오는 것은 어려웠습니다. (Past tense) - It has been difficult COMING here (but I am here now)
Infinitive expresses a generality. Hence, PRESENT ->FUTURE.
여기 오기가 어려울 것입니다 - (Future tense) It is going to be difficult TO COME here (I expect so learning from past experiences).
Gerund and Infinitive are not interchangeable in those cases.
There bound to be other occasions when only one tense may be use in English. But without specific examples, it is difficult to explain.
를 could be in the sentence but just not after the 오.
In this case 오는 is not a noun but an adjective. The 는 there is not a topic marker, it is used to convert the verb into an adjective.
Where you could use 를 is on the 건. 건 is a short form of 것은. Depending on the topic of the sentence and where you want to put the emphasis, you could replace 건 with 것을.
EDIT: see oee16 comment, my advice was not correct in this instance with this particular verb