How would you say I should not have taught you that? Which of the following sentences is correct?
Mi ne devus instruinta tiun al vi.
Mi ne estus devinta instrui tiun al vi.
There are a few ways in Esperanto to express the idea of "should." Often it is with a form of devi.
The conditional -us ending is not a tense, but is rather a mode. It therefore does not express time. So we can use verbs ending in -us to refer to hypothetical events regardless of when we conceive of them potentially happening or potentially have happened.
This suggests using devus in your example.
Note also that devus has a special meaning (according to PMEG) in that it often represents not hypothetical obligation, but rather real obligation unfulfilled.
Often times people, probably due to national language influence, will feel a lack in the timeless Esperanto -us ending and come up with devintus. I do not use that word because I feel it is clunky, and because of an established tradition in Esperanto not to add verb endings to participles. In addition, since -us is timeless, devintus still does not actually express time.
I often encourage people to learn to express ideas and not translate words. If we ask ourselves what the idea is you're trying to express, we can still express the idea without using a form of devi.
If you would like to use devus (to express real obligation unfulfilled) you could say:
I have two versions, I think they are more or less synonymous with a little difference, so how I understand that:
Please correct me if I'm wrong.