首先很感谢网友“好题”对我写的“略谈than引导比较句的还原与省略”的问题的评论，我不得不把这个问题再延伸一步。其实这样意义也不是很大，这个问题历来都是语言学家争论的问题。“好题”网友的意见说直白点就是省略形式“than  expected/necessary”中到底是省略了is还是are. 这个问题的水太深，我今天有点忙，就写点我的理解算是抛砖引玉。

这个问题涉及到两个问题：

1.像expect，report等这些词改写比较句是都有两种写法（这与这些词的用法有关），最终合为一句时有时势必会出现到底是用than was expected还是than were expected。比如：

在“Current English Usage1981修订版”一书的第263页：

[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.

Wood博士认为两句都是正确的。

但是也有不一样的，比如（Quirk1985名著P859）：

The girls swam faster than expected.拆句为：

The girls swam fast.和It was expected that the girls would swim fast.

两句合为一句：（这是Quirk的理解）

The girls swam faster than it was expected that the girls would swim.

The girls swam faster than it was expected that they would swim.

The girls swam faster than it was expected that they would do. <BrE>

The girls swam faster than it was expected that they would.

The girls swam faster than it was expected they would.

The girls swam faster than (was) expected. （这一句就是上面的原句）

Quirk并没有把它理解为：

The girls swam fast.和The girls were expected to swim fast.

合句后变为：

The girls swam faster than （（they） were） expected （to swim）

即：The girls swam faster than （were） expected.

原因是什么？很多书上都没有细致的讨论，也只有Wood博士在“Current English Usage1981修订版”一书的第263页只言片语的说了一下，并没有说原因。我也找了几个例句:

They came earlier than expected. = They came earlier than it was expected for them to come.

They returned home sooner than was expected.（A remedial English Grammar for foreign students by Wood，P74）

从上例句可以看出，似乎在比较要素作状语时，一般都理解为形式主语的改写法，此时用单数was。

如果是充当比较要素的是作表语或是主语等，见如下例句：

It lasted longer than was expected [i.e. than it was expected that it would last].（Modern English Sytax）

However, the relationships are more complex than were expected based on prior research and theorizing.（Journal of Marketing Channels, 20:239–259, 2013）

Although little physical evidence of Adelaide’s engineering heritage remains, the research for the booklet uncovered many more sites than were expected.（Australian Journal of Multi-Disciplinary Engineering, Volume 8, 2010 - Issue 1，75-87）

At the Lord Mayor’s reception there were fifty more people than was expected.（The Macmillan dictionary of English colloquial idioms by Wood，P124）

Indeed, the whole trial could be stopped prematurely if there were significantly more adverse side-effects than were expected.（The American Journal of Bioethics，Volume 11, 2011 - Issue 4，w1-w3）

Fewer came than were expected.（韦氏高阶P610）这句比较要素是主语，但是用的是were，说明不是用形式主语来改写的。

上面的例句表明有些乱，似乎没有什么规律可言。

2.有人会把than理解成关系代词来解释到底是用单数was还是复数are。关于这个方面，赵振才教授在“英语常见问题解答词典第五版”的1477页简单讨论了一下。理解成关系代词确实是一个方法，但不是万能的，有时依然会碰到无法解释的句子。

There is a strong tendency to give patients far more drugs than is necessary.（Longman Collocations Dictionary and Thesaurus, 2013 5th)很典型没法解释,这里很明显比较要素是复数，为什么用is？

The really serious cases are fewer than has been suggested.  没法解释

At the Lord Mayor’s reception there were fifty more people than was expected.（The Macmillan dictionary of English colloquial idioms by Wood，P124）没法解释

结论：

1.如上所述，把than理解成关系代词有些比较句还是解释不了。

2.如果比较要素是用作状语。则用形式主语的改写方法要优选，即用单数is/was。如：

[a] They return home earlier than is expected.（这句优选）

[b] They return home earlier than are expected.

3.其他情况就比较乱，各种情况都有，在专业期刊上这样的例句很多，研究起来比较花时间，有兴趣的网友可以试试。

4.要避免这样很有争议的问题，最好是直接省略成than expected/necessary，这样就没有这些问题了。