Translation:Then the quality will have been proved.
For complex historical reasons, prove developed two past participles: proved and proven. Both are correct and can be used more or less interchangeably (this hasn't been proved yet; this hasn't been proven yet). In British English proved is more common, with the exception that proven is always used when the word is an adjective coming before the noun: a proven talent, not a proved talent (Oxford dictionaries)
Try these pages for an explanation:
This is another passive construction that has made its way into a section on a particular verb tense. I can't "report" it since I didn't get it wrong, but someone really needs to go through and edit these out. Is there a way of alerting the mods/staff other than reporting? There are a few of these.
Based on how they explain it in tips and notes, "must have..." should be accepted here. Duo is inconsistent as to how and when that form is accepted though: it is accepted in some of the exercises, but not all, with no obvious logic. I think it's a matter of reporting it as an accepted response... and then waiting...
"will have" expresses in the future. "been demonstrated" is past anteriore. "must have" is past tense. Similarly, conditional = would + verb. Or, parlare > parler-ei, esti, ebbe = would speak
avr- (future and conditional stem) + ei, esti, ebbe + PP parlato = would have + PP spoken or chiesto, asked dovere, must, have to, ought to avrei dovuto = should have or ought to have (or must have) . If infinitive follows, fare = given or guidare = driven or riccilare = recycled. potere avrei potuto = could have Followed by infinitive ballare = danced volare = flown volere avrei voluto = would have liked Followed by infinitive to drive, to dance
Imperfect (-vo, -vi, -va, -vamo,-vate, -vano) is just past tense, used to + verb or was +verb "ing" (published, used to publish, or was publishing). Avere = ave +vo, vi, va, vamo, vate, vano or Essere = Ero, eri, era, era-vamo, era-vate, era-no ADD PP and = had + PP. Or, era partito/a had left erano usciti/e had left Avevo letto , had read avevo nuotato had been swimming