几道关于时态的题 请教

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5. Although the Supreme Court ruled as long as 1880 that Blacks could not be excluded outright from jury service, nearly a century of case-by-case adjudication has been necessary to develop and enforce the principle that all juries must be drawn from “a fair

6. Developing nations in various parts of the world have amassed $700 billion in debts; at stake, should a significant number of these debts be repudiated, is the solvency of some of the world’s largest multinational banks.

7. For many people, household labor remains demanding even if they can afford household appliances their grandparents would have found miraculous.

这三道题的时态我都不能很好的理解，有人能给讲讲么？

ppp1

个人的一点想法：fficeffice" />

5. 主语中有a century 同样也含有一段时间的意思，所以后面用了完成时。

6. should a significant number of these debts be repudiated 用的是虚拟表一种假设，但主句是一般时，根据题意（主要是前半部分），这种危险是事实存在的，所以没有用虚拟。

7. household appliances their grandparents would have found miraculous. 这里用了表过去的虚拟，说明 household appliances 不是一个新发明，而是已经存在着的东西。