Here's my take, based on, judging by (meaning depends on the word it is paired with)
Expresses that the clause that comes after is something that the speaker has heard, seen, or wishes or believes. Commonly added to verbs like "장담하다," "짐작하다," "보다," "바라다."
Example
맹세코말하건대나는하지않았어.
I swear to you, I didn't do it.
장담하건대그친구가분명히시험을잘볼것이다.
I can assure you that he/she will do well on the test.
지금까지받은보고를종합적으로판단해보건대부검할필요는없을것같습니다.
Based on a comprehensive look at the reports I've received up until now, there should be no need to perform an autopsy.
억양으로미루어보건대외국인이다.
Judging by his/her intonation, he/she is foreign.
생각건대인생은꿈에지나지않다.
I think that life is nothing more than a dream.
추측건대이실험실에서탈출하려면초록문을열어야해.
I predict that we will have to open the green door to escape from this facility.
말투로짐작건대지금아내와사이가별로안좋은것같다.
I gather from his words that his relationship with his wife is not currently at its best.
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