3 to have done 用于表示将来完成?
这个句子是某美国报纸2015年时的新闻报道语句：By 2019, they are projected to have fallen off by roughly a third since their peak in 2009.
我知道这句话的意思应该是: By 2019, they are projected to fall off by roughly a third since their peak in 2009.
为什么这个地方要用to have done呢？一般to have done都是用于类似下面的句子中：
The earthquake is reported to have claimed more than 300 lives in Indonesia.
麻烦解释一下。谢谢！
最佳答案 2019-04-02 16:08
By 2019, they are projected to have fallen off by roughly a third since their peak in 2009.
这个句子语法有问题。新闻用语力求简洁、迅速出稿，难免出现语法错误。by 2019这个时间状语并不是修饰谓语动词are projected的，而是修饰不定式的。这个句子本来应该是下面这样的：
It is projected that by 2019 they will have fallen off by roughly a third from their peak in 2009.
在做句型转换时，从句的主语提升为主句的主语，从句的谓语改为不定式, 不带to的不定式改为带to的不定式，完成体不变。但原来修饰从句的时间状语by 2019无处可放了。无奈之下，作者将其放在句首。但句首的时间状语应该是修饰谓语动词的，而不是修饰不定式的。所以这个句子不应该用提升主语结构，而应该用It is projected that句型。
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其它 2 个回答
“截止到......为止，动作完成得怎么样了”本来就是完成时的核心概念。所以不定式的完成时，表示在某特定时间，动作已完成。原句特定时间是“2019”，句子强调截止到2019年，会下降1/3，也正因为强调这个特定的时间，所以by 2019才会置于句首。
to have fallen off 属于将来完成时态用法，相当于will have fallen off
By 2019, they are projected to have fallen off by roughly a third since their peak in 2009.
=By 2019, it is projected that they will have fallen off by roughly a third since their peak in 2009
=By 2019, people project that they will have fallen off by roughly a third since their peak in 2009
预计到2019年，这些数字将比2009年的峰值下降约三分之一。
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