It's not that it changed, as the proponents of the view you learned in grade school have all died out. There never was a speech community that followed those rules; they were dreamed up by country parsons with classical educations and too much time on their hands, who felt that The Poor would do better if they talked more like their Betters did, or at least like the parsons thought the Betters would speak if they tried hard enough. Basically they were Influencers, as we call such people today. I learned this stuff in school too, but I never heard anybody talk that way. [JL]
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As far as the history of would and should is concerned, there used to be English modal verbs that were inflected for tense. Will (actually willan in OE) was one, and it had a past tense would; shall was another and it had a past tense should. But now they are just four of the 9 English modal auxiliaries and don't inflect for tense any more. Nor have they ever commuted with one another in the manner you describe (btw, the original zombie source says that to emphasize, one reverses the pattern -- another thing I've never witnessed). [John Lawler]
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Fowler in 1908 struggled to explain the usage of shall and will, suggesting the existence of 3 systems and 7 rules governing their use. "Rule 2. The Coloured-Future System" appears to match the OP's contours. I've struggled and failed to summarise or indeed understand Fowler's remarks. Perhaps someone else could attempt it. Regardless, an excellent rule in life is to ignore anything a [*non-expert] tells you. [Stuart F] [*sanitised]