passive forms

[arslongus]

The sentence Res publica consiliis meis eo die ex igne atque ferro erepta est
I translated as:
On that day, the commonwealth from the fire and sword by my counsels been been snatched away.
but Benissimus translates as
On that day, the republic was rescued from the fire and the sword by my plans.

My question is not with synonyms, or word order, but with the passive form. In what case is the construction of passive participle and auxiliary verb treated such that erat retains its normal meaning of 'was' and the participle functions adjectivally? The alternative that I have in mind is the one that is set forth in ch. 19.
Strictly speaking, it seems that to say 'was rescued', (a perfect indicative passive form), one would use erepta est, and that erepta erat is in fact the pluperfect passive. I've tried to keep these straight by thinking that with passives, the forms of esse move over one spot to the right, so to speak. That is, when combined with the passive participle, sum >>> I was, eram >>> I had been.
Of course, B could also simply be translating somewhat freer than I am reading it

[nostos]

The sentence Res publica consiliis meis eo die ex igne atque ferro erepta est

My question is not with synonyms, or word order, but with the passive form. In what case is the construction of passive participle and auxiliary verb treated such that erat retains its normal meaning of 'was' and the participle functions adjectivally?

The alternative that I have in mind is the one that is set forth in ch. 19.
Strictly speaking, it seems that to say 'was rescued', (a perfect indicative passive form), one would use erepta est, and that erepta erat is in fact the pluperfect passive. I've tried to keep these straight by thinking that with passives, the forms of esse move over one spot to the right, so to speak. That is, when combined with the passive participle, sum >>> I was, eram >>> I had been.

Of course, B could also simply be translating somewhat freer than I am reading it

arslongus, I think you've answered your own question. The passive participle combines with esse to form pluperfect/perfect tenses, and yes, it does 'move one over to the right'.

The perfect can be translated (either is equally correct, just depends on context which unfortunately the P&R and Sent. Ant. don't give):

erepta est: have been rescued/snatched away (perfect passive)
erepta est: was rescued/snatched away (also perfect passive, just depends on which feels better contextually)

As for when the participle functions adjectivally, that depends on context too: and it means 'having been rescued' or just 'rescued' when it's used adjectivally, eg res publica erepta erat (est) bona: the rescued state was (is) good OR the state having been rescued was (is) good.

Sorry I didn't come up with a better example but I think this one illustrates the point.