有同学问了一个关于“It's time句型”的问题，如下图所示：
If you have really been studying English for so long, it's about time you _____ able to write letters in English.
A. should be B. were C. must be D. are
问：我选了A，错了，答案是B，请问A和B两个选项区别在哪？
答： 请先参考下面例句：
1.It's time that we get back to some core principles that united Republicans in the past.
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2.So I think it's time that the people know what they've been up to.
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3.So we've heard conflicting evidence, conflicting statements, and it's time that we get to the bottom of it.
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你给的例句中从句是真实条件从句，主句应该是真实结果主句，那么正确的答案应该是D。
如果结果主句是虚拟语气，那么条件从句应该改为非真实条件句，那么条件从句就应该用虚拟语气： if you had really been studying......，那么较为合适的答案应该选择B。
通过以上分析，我们可以看出It is time这种句型后面既可以接虚拟语气，也可以接真实语气，这点和我们中文是一致的。
如：
1. It is high time that someone gives him the punishment he deserves!
早该有人给他应得的惩罚了。
2. it is time that we gave them a chance.
是我们给他们一个机会的时候了。
最后总结， It is time句型既可以表达虚拟，也可以表达真实，用过去形态表达虚拟，用现在形态表达真实。
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