Question

如果是It＋be动词＋adj＋that从句，be动词为过去式或将来时或完成时，那从句用什么时态？

概念有点不清楚……
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Answer 1

要看具体语境。
It is/was ＋adj./n.＋that主语从句多数情况下用虚拟语气，表达一种要求，命令或者责备，建议，或主观看法。that 从句用“should+do,英国英语可以省略should”e.g.It is important that he (should) go to school.
但是如果只是表示陈述一个事实而不是强烈的感情，完全可以用一般的陈述语气。
It is strange that he had made a mistake.（陈述一个事实：他犯了错误让人有点奇怪。）
It is strange that he (should) have made a mistake. （虚拟语气，说话者是在责备他本不应该犯错。）
e.g. It is a shame that the rain spoiled our picnic. \It is a pity (that) he lost so much money. \He was a very brave man and it is surprising that he lived so long. \It is strange that nobody knows where he lives .
特别注意that从句谓语动词用“(should) + 动词原形”的情况：
1."It is important (necessary, strange, impossible, natural) that .... "
e.g.It's necessary that we should have a walk now. \It's natural that she should do so.\ It's important that we should take good care of the patient.
2."It is a pity (a shame, no wonder, better) that .... "
e.g.It was a great pity that you (should) have made so many mistakes in your math exercise. \It will be better that she stay with her grandmother.
3."It is suggested (requested, proposed, ordered) that .... "
e.g.It was suggested that the sports meet be put off until next Saturday. \It is desired that we (should) get everything ready by tonight. \It was ordered that the medicine be sent there by plane.

Answer 2

看句意来的 看定语内容在过去发生就一般过去。在过去发生对过去有影响就过去完成。如此云云

简单地说就是和be动词时态没关系。看句意啊句意！
譬如：It was impossible that he had changed his dejected life and made the world moved.
It is impossible that he has done this tough work.
It was impossible that he would do this tough work.
It was impossible that he was willing to accept the truth.
初三党正在备战中考。 希望能帮到亲哇咔咔！~~~