Zuwena网友在论坛上提出了关于“than引导比较句的还原与省略”的诸多问题（https://www.cpsenglish.com/question/47242），我觉得是很好的问题，也很有意思。我也简单说说。我就不一一回答了，曹老师也回答了一部分，我说些我的拙见：

能改写成形式主语it的限于一些词，如：normal/usual/possible/necessary/expected等等很多。而且即使能改写成形式主语的形式，有时也很尴尬，因为其逻辑主语不知道性别导致人称代词不知道是用哪个，比如：

The boss made more mistakes than usual.

拆句：

The boss made some mistakes.

It was usual for the boss to make some mistakes.

由于不知道性别没法确定是him还是her。所以：

合一句：

The boss made more mistakes than (it was) usual (for her/him to make).

也可以写为：The boss made more mistakes than (was) usual.

上面的括号部分都是可以不要的。

也就是说只能这么写了。

还有没有主语的情况，比如祈使句：

Don’t spend any longer on it than (is) necessary.

拆句如下：

Don’t spend that long on it.

Spending that long on it is necessary.

合为一句：

Don’t spend any longer on it than (is) necessary.

这样理解比形式主语的理解要更简单一点。

这是剑桥英语语法的作者Huddleston的做法，这个做法的好处是不需要关注主语，也不担心主语到底是him还是her还是you等等。很值得学习。

你提到的：

第二种补全方式：

I buy more books than are necessary.

I buy many (A) books.

Many (B) books are necessary.

A>B

I buy more books than are necessary.

这种不用形式主语的写法本来是没问题的，问题出在这一句：

Many (B) books are necessary.

应该是Buying many books（B） is necessary.你把重要的信息弄掉了。

这样就跟之前的形式主语的改写一致了！

至于比较分句中的形式主语的省略问题，其实在Quirk的1972版的“A Grammar of contemporary English”的第769页有讲：

Further, when the comparative clause contains an anticipatory it construction,not only is the whole nominal clause removed, but the it as well.

奇怪的是在Quirk的1985版名著“A Comprehensive Grammar of the English Language”却没有这句描述。

我的感觉是Quirk在写上面这本书中关于比较句的章节时有所顾忌，似乎在刻意避免这种形式主语的写法，顾虑的原因可能是上面说的逻辑主语很多时候是不能确定到底是哪个人称代词，改写很尴尬。

而上面提到，Huddleston用了另一种方法改写很好的避免了这些情况。

你提到的：

第二种还原方式：

There were many (A) casualties.

Many (B) casualties were reported.

A>B

There were more casualties than were reported.

原句是：

There were more casualties than was reported ( = it was reported there were).

这一句的原出处是Wood博士的“Current English Usage1981修订版”一书的第263页。Wood博士认为没有省略之前是上面括号里面的，也就是说用was。顺便说一下，张道真的“现代英语用法词典修订版”的第1614页也收录了这个例句。

我支持Wood博士的看法：改写成it形式主语的写法肯定是没错的，而改写成Many (B) casualties were reported.的问题在于将原句中的部分信息丢失。

你提到的：

类似的两种方式补全的还有：

句1：They return home earlier than is expected.

句2：They return home earlier than are expected

在这个问题上，我同意你的理解，上面两个句子都是正确的，区别只是在于不同的理解得到了不同的形式。其实说白了，这主要是与expect这个词的用法有关，如果省略句1的is，句2的are，两句形式上就完全一样了，而事实上，than expected确实要比than is/are expected要更常见。

还是在“Current English Usage1981修订版”一书的第263页，Wood博士也举了一个这样的例子：

[a] There were more people present than were expected, if the sentence is short for . . . than there were people expected.

[b] There were more people present than was expected, if it stands for than it was expected there would be.

两句都是正确的。

不过我对句a的理解与Wood博士的不太一样,省略后形式就一样了：

There were more people present than were expected (to be there).