Can 'should' be used in the past? I don't mean a regret here. I haven't found an answer on the Internet because all the explanations describe regrets.

I didn't know what I should/was supposed to do

  • Both are fine, as would be I didn't know what I should / was supposed to have done. Note that in your case we're pairing [negated] know + do, but in other contexts it might be the same verb both sides of "should / was supposed to" - He didn't do what he was supposed to do. In that case, most native speakers would "delete" the predictably-repeated do at the end (but there's no such "deletion" if it's "backshifted" to ...to have done). Jul 6 at 12:20
  • @FumbleFingers thank you for your comment, but my question was rather about 'should' vs 'supposed to' and the difference between them if any. On the other hand, your comment made me think about the difference between 'do' and 'have done'. I feel that 'have done' expresses regret, but there's no regret here. It's about the speaker's thoughts back then, not about what they were supposed to have done but didn't. Am I right?
    – i_yre_b
    Jul 7 at 11:10
  • If you're actually only interested in the choice between should and [be] supposed to, I don't see why your example context includes I didn't know. I seems to me you could have just asked about the difference between I should go and I am supposed to go Jul 7 at 11:51
  • @FumbleFingers please, read the question again. I asked whether it can be used in the past.
    – i_yre_b
    Jul 10 at 11:28
  • Ah, I see. Well, there are lots of written instances of knew what he should do as well as knew what he should have done (the Past Tense versions of I [didn't] know what I should do / have done). I'd say should is a "degenerate" verb that doesn't have a Past Tense form, but hopefully you can see how that aspect is conveyed through the auxiliary forms do / have done. Jul 10 at 12:19

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