I would have said "it proves, it proved, it has proven" were the principal parts of to prove. Are we not doing that anymore?
Both "proved" and "proven" are in use as the past participle; a couple of dictionaries I checked briefly show that "proven" is marked as "especially US English".
Both versions are accepted here in translation.
The -pu' is not allowed here. Type 7 suffixes are not allowed on the second verb of a sentence-as-object construction.
moH yIH net tob One proves that the tribble is ugly is the best you can do.
Thanks for the reminder!
I've "deleted" this sentence and added moH yIH net tob with gloss "It was proven that tribbles are ugly". Your translation "One proves that the tribble is ugly" is also accepted.