Hello everyone! "Only when darkness had again descended upon me should I realize how much I had left unseen. " (From Three Days to See) Could you please tell me about this sentence that Why is there "had descended" being used with "should I realize" instead of "should I have realized"? I've learned that when the clause uses "had done" to assume about the past, the main clause is gonna be "should have done" or "would have done". (Is this right fundamentally?) Then why the sentence have the organization like that? I would appreciate it very much if you could do me a favor.