건데

'Speaking as one who…, I say'

-건대 introduces the ground you are speaking from before you deliver your verdict, and it comes in a small set of fixed phrases: 보건대 'judging by', 장담하건대 'I guarantee', 예상하건대 'my guess is'. 내가 그 쯔양의 실력을 보건데... 'judging by Tzuyang's ability...' It is formal and a bit oratorical. Note the correct spelling is -건대, though subtitles often write 건데; two lines here (이건데, 끝이건데) are unrelated.

Register: formal, oratorical