求助关于conditional construction 的问题

fujiyukiko

本菜鸟对于conditional construction很是困惑，到底是什么东西，什么时候用啊？翻了manhattan也没找到，好像只在if...then 表示unlikely case in the future的时候提到要用if hypothetical subjunctive+then conditional，但是看到og13 42题彻底懵了To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.

A. have to know a great deal more than now about the social and economic
B. have to know a great deal more than they do now about the social and economical
C. would have to know a great deal more than they do now about the social and economical
D.would have to know a great deal more than they do now about the social and economic
E. would have to know a great deal more than now about hte social and ecomomic

答案是D，解释说 this sentence explains a hypothetical situation and therefore calls for a condtional-or contrary-to -fact--construction,because in order to more accurately predit population, demographers would have to know more than they presently know.

这里是manhattan 里面说的unlikely in the future的情况吗？但是没有if hypothetical subjunctive啊。。。 前面那个to...是省略了什么东西吗？好抓狂啊，求高手解释一下什么时候要用condtional construction？？以及这道题为什么要用would？？谢谢啦

bigfatwhiterabb

这里是从语义判断的，为了怎么怎么，科学家可能要怎么怎么。to develop是不定式表示目的。