各位老师好，因为担心内容太长，下面内容本来是发在“文章”栏目的，但经过管理员提醒，重新发在“问答”栏目。恳请各位老师指教。
各位老师好，因为最近遇到所谓的“准关系代词”，在搜索相关信息的时候就看到了网站上各位老师关于than用法的回答。根据各位老师的回答，我整理了一下我理解的部分和我的问题，希望老师指正。
我的办法是通过拆解、分解句子的方式来理解than引导的比较分句。
前面几个比较容易：
Tom is tall (tall的程度是A).
Bill is tall (tall的程度是B).
A> B
所以：Tom is taller than Bill is. 这样“than Bill is”的分句里面，其实than（即曹老师命名的比较代词）做了is的表语，也就是说than其实是承担了比较项的句子成分，这个句子也是完整的，并不是被省略的结果。
下面的例子也可以用拆解的方法来理解，因为这个than后面的句子用了it做形式主语，所以省略掉真正主语和形式主语it之后，就能得到给出的句子：
Those countries are richer than appears.
Those countries are rich (rich程度是A).
It appears that those countries are rich (rich程度是B)
A> B
Those countries are richer than it appears that those countries are (are的表语、也就是比较项rich的成分被than充当了).
在这个句子的基础上，省略掉真正主语和形式主语就会变成：
Those countries are richer than appears.
但是这里是我的第一个问题，似乎只要有it做形式主语的话，it和真正主语都可以省略，是不是真的有这样的一个规则呢？
类似的例子还有：
Don’t give children more money than is necessary.
补全形式是：Don’t give children more money than it is necessary to give them (give 的直接宾语money作为比较项，其成分被than充当了).
He drove faster than was wise.
原句是：He drove faster than it was wise for him to drive (这个drive的状语fast作为比较项，在分句中的成分被than承担了)。
但是有的情况下，than分句的补全似乎可以采取其他形式，一旦采取了其他形式，than分句的谓语动词用单数还是负数似乎就有些问题。
比如下面的句子，如果用it做形式主语来补全，那么用is；但是第二种补全方式为什么不对呢？
I buy more books than is necessary.
I buy many (many程度为A) books.
It is necessary to buy many (many程度为B) books.
A> B
I buy more books than it is necessary to buy. (than is the object of buy)
I buy more books than is necessary.
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.
我的第二个问题就是，这个第二种补全方式为什么是错的呢？
如果第二种补全方式也可以接受，那么下面这道题其实选were也可以：
之前有同学提问的：“There were more casualties than reported.
A was B were”
第一种补全方式：
There were many (A) casualties.
It was reported that there were many (B) casualties.
A> B
There were more casualties than it was reported that there were.
There were more casualties than was reported.
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.
类似的两种方式补全的还有：
They return home earlier than is expected.
第一种补全方式还是it在than分句中做形式主语：
They return home early (程度为A)
It is expected that they return home early (程度为B)
A>B，也即A早于B
They return home earlier than it is expected that they return home (比较代词than做了分句中真正主语里面return的状语，也是整个比较结构比较项)
They return home earlier than is expected.
但是这个句子可以有另一种补全方式：
They return home early (程度为A)
They are expected to return home early (程度为B)
A>B，也即A早于B
They return home earlier than they are expected to return home.这个句子可以省略到什么程度呢？因为句子主语和than分句主语是一致的，可不可以把than分句主语they省略掉呢？变成：
They return home earlier than (they) are expected (to return home)/(to do)
= They return home earlier than are expected (to return home)/ (to do).
与上述“省略共同主语”相关的，是我的第三个问题。
这个例子也是之前同学提问过的：
The result will be less effective than would be achieved by a teacher in harmony with the unit.
按照曹老师的回答（https://www.cpsenglish.com/question/43690）
补全形式应该是：
The result will be less effective than (what) would be achieved by a teacher in harmony with the unit (is) (并且比较项是is的表语，即effective，在than分句中，than充当了这个成分).
这样一来，省略了从句主语的一部分what和谓语is。
在上面的回答链接里面，曹老师也提到了这种省略似乎没有明确的规则可寻（类似的可以用what来补全句子的例子，还可以在章振邦《新编高级英语语法》（2019版）下卷第1255页上找到只不过作者将其标记为“很难具体指明省略了何种成分的”的例句，比如“The coffee here is weaker than I like.”）
但是，我按照上面“省略共同主语”的例子“They return home earlier than are expected”，我们是否可以这样理解：
The result will be less effective than (the result) would be (if the result) achieved by a teacher in harmony with the unit.
The result will be less effective than (the result) would be (原本的比较项effective其实应该在would be后面，但是被than承担了成分；而且would be是虚拟语气，与现在事实相反的主句) achieved by （这个achieved by不再和would be连在一起，而是过去分词做状语，因为achieved by的主语和整个句子的主语一致，都是result，所以被省略了）.
类似的，我仿照the result will be less effective又写了另一个句子：
He was shorter than could have been raised by his own parents.
这句话是仿照the result will be less effective来写的，而且这个句子能更明显看出虚拟语气的使用。
He was short.
He could have been short （事实的身高比这个虚拟假设的身高要更矮） if he had been （用had been是因为需要使用与过去事实相反的虚拟语气） raised by his own parents.
He was shorter than (he) could have been (if he had been) raised by his own parents.
最后第四个问题，还有些零散的例子也希望老师看看是否正确：
1. More books are bought than are needed.
Many(A) books are bought.
Many(B) books are needed.
More books are bought than are needed. (than 是分句主语，也是比较项。)
2. Those countries are richer than he estimates. (这个句子是我自己仿写的)
Those countries are rich (A)
He estimates those countries to be rich (B)
Those countries are richer than he estimated them to be.
Those countries are rich (A)
He estimates those countries are rich (B)
Those countries are richer than he estimates those countries are.
=Those countries are richer than he estimates.
3. My grade is higher than he achieves.
My grade(A) is high.
He achieves a high grade(B)
A> B
My grade is higher than (what) he achieves (is这个is后面的表语high被比较代词than充当了，这个句子和result will be less effective类似).
4. The flower is beautiful this year.
The flower was less beautiful last year.
The flower is more beautiful this year than the flower was last year.
那么这句话可以省略到什么程度呢？
The flower is more beautiful this year than was last year （因为than分句的主语也是the flower是否可以省略呢？）
5. The results are good (good的程度为A).
The results are expected to be good (good的程度为B).
The results are better than the results are expected to be.
=The results are better than are expected to be.
=The results are better than expected.
6. Many (A) people turned out.
Many (B) people were expected to turn out.
More people turned out than were expected to turn out.
=More people turned out than were expected (to do).
Many (A) people turned out.
It was expected that many(B) people turned out.
A> B
More people turned out than it was expected that turned out.
(主语从句中的主语，作为比较项由than承担了，不能重复出现)
=More people turned out than was expected.
Many (A) people turned out.
He expected that many(B) people turned out.
A>B
More people turned out than he expected that turned out.
(expect的宾语从句中的主语，作为比较项由than承担了，不能重复出现)
= More people turned out than he expected.
Many (A) people turned out.
He expected many(B) people to turn out.
A>B
More people turned out than he expected to turn out.
= More people turned out than he expected.
