To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.
答案解释，后面必须用conditional verb: would have to know, 因为前面是假设的情况。
但是to do sth 可以表示目的啊，为什么一定是假设的，不可能的情形呢？
谢谢！！
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发表于 2014-10-9 21:46:19
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