The passive we have in Germanic is pretty limited. It only exists in Gothic as a pure tense; other languages like Old English had a few relics of passive verbs that gradually got assimilated as active (cf OE ic hātte, vs OI ek heita – the former is a passive construction, the later is active). So what we’re able to determine as a passive paradigm in Proto-Germanic is the present indicative and present subjunctive (or optative, but that’s another story). Note: Endings are the same for both strong and weak verbs. There is no record of any dual forms.
1s eka berade > ik baírada
2s þu nasjaze > þu nasjaza
3s his hauzijade > is hausjada
pl jus sehwande > jus saíƕanda
1s eka berēdau > ik baíraídau
2s þu nasjēzau > þu nasjaízau
3s hita berēdau > ita baíraídau
pl wīs sehwēndau > weis saíƕaíndau
If you look at the passive in IndoEuropean, you can see that Germanic has already lost quite a lot of this paradigm. IE had a few different types of passive: central, peripheral, and secondary. More specifically, these are areal terms - Germanic, Greek, Iranian, Slavic, and others took the central endings, while Latin, Celtic, Hittite, and Tocharian took the peripheral "r" endings. Here’s the passive central paradigm from IE for baíran < berana < bʰéronom:
The subjunctive/optative is exactly the same, but the second vowel is long (cf ablaut). The reason the subjunctive is so different in Germanic is because the particle –u was added to the end of the subjunctive (and various other things, as late as the Gothic era) to indicate that it was in question. (Hence, the –u particle in gothic to form an interrogative.)
Forgive the length of this – I’m not so much composing a post here as thinking this out on paper for myself.
So moving on, all of the above is what we already know about Gothic. What we want to know is what the past passive forms might have been if they even existed at all. Personally, I believe that they probably did not exist by Gothic times, and that past passive was either not used in the way we think of it today, or a compound tense was used as in the modern Germanic languages. But let’s speculate for a moment on what a past passive might have been.
If you look at the aorist mediopassive for bʰér-, it doesn’t differ from the present enough that they would have been distinct in Germanic. But Germanic had already come up with a nice solution for that – the “dental past.” I think that probably any past passive that might have been used would have taken a dental past for weak verbs, and just the past ablaut form with passive endings for strong verbs. So you might end up with something like this:
1s eka hauzidade > ik hausidada
2s þu hauzidaze > þu hausidaza
3s hita hauzidade > ita hausidada
pl hīs hauzidande > eis hausidēdanda
1s eka barade > ik barada
2s þu baraze > þu baraza
3s his barade > is barada
pl hīs bērande > eis bēranda
I’m totally making this up, so feel free to offer other suggestions. I think it would be plausible, though, and as I said, I don’t think they actually used a past passive in Gothic.
Oh, and let’s not forget the subjunctive:
1s eka hauzidēdau > ik hausidaidau
2s þu hauzidēzau > þu hausidaizau
3s hita hauzidēdau > ita hausidaidau
pl hīs hauzidēndau > eis hausidēdaindau
1s eka barēdau > ik baraidau
2s þu barēzau > þu baraizau
3s his barēdau > is baraidau
pl hīs bērēndau > eis bēraindau
And since I’m totally making things up at this point, I’m also going to say that, since we have no record of the dual forms in the passive, they probably assimilated the plural forms, which seems like a natural progression. (They eventually did so in all other Germanic languages.)
Ƕas þagkjiþ jus?