quippe Drusi magna apud populum Romanum memoria, credebaturque, si rerum potitus foret, libertatem > redditurus> ; unde in Germanicum favor et spes eadem.
The commentary says we should supply fuisse with redditurus. I’m not clear why it’s not redditurum. We have a conditional sentence depending on credebatur, so it becomes oratio obliqua. Unreal protasis remains the same, the apodosis goes into infinitive. But why does the participle, being part of an infinitive, remains in the nominative? Just because the sentence depends on a passive verb? I cannot find an explanation for that.
He’s the subject of credebatur (“he was believed …”), so the predicate redditurus is in agreement with the subject and remains nominative. Only if credebatur were impersonal (“it was believed that he …”) would an accusative come into play (eum redditurum).
I can only suggest you read my reply again, my friend. Compare e.g. videtur libertatem redditurus esse (“he seems …”), or non dicor stultus esse (“I am not said …”), perfectly regular Latin. See Gildersleeve & Lodge §528. It’s true that in classical prose creditur would usually be impersonal, but this is Tacitus.
I don’t see why you need to be rude. I’m not your friend. I hear Americans are generally too solipsistic to realize their back-slapping attitude tends to grate on people of less degenerate cultures. Is this true? I’m telling you now that it is unwelcome.