I figured I’d just try to handle one example of a ずる・じる verb here since I didn’t tackle those in the post I mentioned.
In traditional Japanese grammar, these verbs are classified separately as サ変 (サ行変格活用=‘the sa row irregular conjugation’), which is the same group する belongs to. Here are the forms for 信じる according to Wikipedia:
信ずる(口語)
語幹:信(しん)
- 未然形 (irrealis, used for negation, among other things): -じ (-ない、-よう、-ず、-させる、-られる)、 -ぜ(-ず、-られる、-させる)
- 連用形 (the verb-joining form you see in compound verbs): -じ
- 終止形 (the sentence-final form at the end of sentences): -ずる、-じる
- 連体形 (the noun-joining form that you see in relative clauses e.g. 私が信じる友達): -ずる、-じる
- 仮定形 (the hypothetical form that you see with ば): -ずれ、-じれ
- 命令形 (the imperative form for giving orders): -じろ、-じよ、-ぜよ
The versions that aren’t so common today (mainly the ぜ and ず forms) are probably more traditional. I don’t know enough to be certain of what the set of ‘traditional conjugations’ was. I presume it’s probably the same as what we have for する now (i.e. you do exactly the same thing but replace all the S sounds with Z sounds).
In any case, it’s perfectly fine to stick to 〜じる only for these verbs nowadays, or so I believe.
So it was たい after all for passive “general” one…