I. TheoryThe past conditional, the past form of the Subjunctive II, is used to convey actions that would have occurred in the past - as the conditional past does in English. In English, however, we form this tense by inserting the verb have between the modal would and the past participle (eg. I would have gone, I would have seen, etc.).
In German, the modal verb würden is not used in this construction, but rather the auxiliary verb (sein or haben) uses its own Subjunctive II form and is then combined with a past participle.