틀림없다 I know the word 틀림없다 means "to be free of mistakes, to be certain, to be correct", and that it can also mean "it must be". So I have seen the following expression with a verb (as a noun): 악몽과 같았음에 틀림없다 It must have seemed like a nightmare. = It must have been a nightmare. That's a verb converted to a noun using the (으)ㅁ form with the particle 에. But then I see it used with -는 것: 매우 신나는 것임에 틀림없다. 1) Why does the noun 것 need to have 이다 + (으)ㅁ added when 것 s already a noun? Why not just: 매우 신나는 것에 틀림없다. Finally, I see other versions, without 임 and with the subject marker 이 instead of 에: 매우 자랑스러워 할 것이 틀림없다. 행복한 것임이 틀림없다. 2) Are they all correct and natural? Is it just up to preference? 감사합니다!