I learned it from this guide, detailing 「だめ」,「いけない」 and 「ならない」constructions.
I was able to assume that the past-tense of "I must not do" is て-formはだめだった (correct me if I'm wrong), but how can I do the same for things I must do? I guess maybe it's negative て-formはだめだった, but as Tae Kim says, that's super long and thus would probably sound weird if I used it outside formal situations. Up to now, I've used the "replace なくて with なくちゃ" slang form, but how would I put that into past tense?
I was thinking about using 行けないだった, but that's even longer than だめだった so it's a no go, I'm guessing. Any advice?