It has been estimated that the weight of all the insects destroyed by spiders in Britain in one year would be greater than the total weight of all the human beings in the country. (New Concept English, Book 4, L.2)
[原译]曾作过这样的估计，在英国，一年中所有被蜘蛛消灭的昆虫的重量，将比这个国家所有人的总重量还要重。[《新概念英语》(四)，安徽科技出版社，p.19]
原文中的 would be 是过去将来时吗？又，estimate 的宾语从句是否永远要求用 would?
would 和 will 一样, 都可用来表示“现在推测”。请比较: That is his mother. That will (would) be his mother. (见 will , will have done, would )。由于主句谓语用了 has been estimated(估计), 所以其宾语从句使用表示现在推测的 would 在逻辑上和惯用法上都是讲得通的。
请看下面的托福题：
It is estimate that a round-trip flight to Mars would take more than a year and a half. (TOEFL)
(应将 estimate 改为 estimated)
从你所提问的句子以及上面的托福题不难看出,这里的 would 不可能表示过去将来时,因为在你提问的句子里,主句谓语是现在完成时,而在上面的托福题里,主句谓语是一般现在时。
但是,应当注意,estimate 的宾语从句并非一定要求用 would。请读下面的例句:
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The archaeologist estimated that the vase was 3,500 years old. (FWF) 考古学家估计这花瓶有 3,500 年的历史。
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I estimate it will take three months to build the boat. (JED) 我估计造这条船要用三个月。
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It is estimated that 25 million school lunches are sold each day. (WTYB) 据估计每天能卖出 25,000,000 份学校午餐。