有语法基础的同学都知道，英语句子除祈使句 (imperatives) 外，都要有主语。
有些句子的主语非常奇怪，叫“假主语”（或形式主语），因为真正的主语非常长，为了避免头重脚轻，需要放在句中或句末。
这种假主语的英文叫 preparatory subjects 或 dummy subjects（dummy 这个词用得很形象）。
英文所用的假主语有二：There 和 it。
There 在 There is (There’s) 和 There are 结构中是当作假主语，真正主语是 There is 或 There are 后面的名词或名词短语。
There’s a girl on the phone for you. (有个女生打电话找你)
这句的 There 是假主语，真正主语是 a girl。
It 在句中当假主语时，真正主语有三种型态：不定式主语、动名词、以及 that 从句。例如：
It’s very hard to believe that he is an honest employee. （很难相信他是个诚实的员工。）
这句的 It 是假主语，真正主语是不定式短语——to believe that he is an honest employee。
如果按常规语序，这句可改成 To believe that he is an honest employee is very hard.。
再来：
It’s worth calling him to clear up the misunderstanding between you and him. （值得打电话给他澄清你和他之间的误会。）
这句的 It 是假主语，真正主语是动名词短语——calling him to clear up the misunderstanding between you and him
按常规语序是 Calling him to clear up the misunderstanding between you and him is worthwhile.。
再来：
It is difficult (that) they would finish their work in two days. （他们要在两天内完成工作是有困难的。）
这句的 It 是假主语，真正主语是that 引导的名词短语—— that they would finish their work in two days。
所以，若按常规语序，这句要写成：That they would finish their work in two days is difficult.
对了，如果真正主语 (即 that 所引导的名词从句) 置于句首，则 that 不能省略。
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