既然你已经看过P1110的内容，想必也知道than后面的是个形式主语it的从句部分省略罢了，此时当然用单数形式，不多说。

要写成appear也是可以的（appear这个词除了用形式主语it也可以用名词作主语），只是比较对象不要省略，即they，改成：

The difficulties are even greater than they appear (to be) at first sight. 就可以，to be可以省略。例如：

1 If you say that there is more to something or someone than meets the eye, you mean that they are more complicated or more involved than they appear to be at first.(Collins COBUILD Dictionary of Idioms Helping learners with real English by John Sinclair1995版)

2 The person is less honest or moral than they appear to be.(同上)

3 As that example suggests, phrasal verbs are often less complicated than they might appear.（The Good Grammar Guide书P95）

4 The facts may well be more murky than they appear at first sight, however.(Colloquial english structure and variation书P124)

alex119 老师认为可以不多说的地方，其实正是诸多网友不理解的地方，需要加以说明的。我对这一部分做一些补充。

词典和语法书都认为than引导比较分句时是连词，不在分句中做成分。同时比较分句通常采用缩减形式，即分句尽量采用省略和替代。关于省略有一条很重要的规则是国内语法教学所忽略的，即比较分句可以省略与主句相同的主语和谓语动词，例如：The river is deeper in summer than (it is) in winter. 但却不能只省略主语却保留谓语动词，例如：The river is deeper in summer than is in winter. 这个句子than后面为谓语动词is，却没有主语，即省略了主语，却保留了谓语动词，这就是语法错误了。改正的方法有二，要么把is也省略，要么加上主语it。英语学习者应该首先记住这条语法规则。然后再进一步了解这条语法规则有二个例外，必须也记住这二条例外：

第一：当比较分句的主语为比较对应项时，主语必须省略，保留谓语动词。例如：

More people came to the party than (had been) invited. (than和had之间不可以出现主语）（had been可以省略是另一回事）

第二：当比较分句的主语为形式主语it，真正主语that从句或不定式短语外置时，可以同时省略形式主语it和外置的that从句或不定式短语。（但不可以只省略外置主语而保留形式主语it。）例如：

Tom is stronger than (it) is believed (that he is).

形式主语it和真正主语that从句要么同时省略，要么同时保留，不可以只省略其中之一而保留另一个。省略后则为：

Tom is stronger than is believed. (is可以进一步省略，这是另一回事）。

Tom is stronger than it is believed. 为错误，it为形式主语，真正主语省略后，形式主语必须同时省略。

现在看网友这个句子：

The difficulties are even greater than appears at first sight.

than后面的appears为谓语动词，比较分句没有主语，说明主语省略了。只有二种情况可以保留谓语动词而省略主语，即以上所说的二种例外情况。本句中的比较项为形容词greater，不可能是比较分句的主语。所以这不属于第一种例外。从appears的单三形式可知，省略的主语是形式主语it，连同真正主语that从句。补全这个比较分句则为：

The difficulties are even greater than it appears at first sight that they are.

即从句的主语并不是the difficulties, 因为the difficulties并不是比较项，其在从句作主语的话是不可以省略主语而保留谓语动词的。由于appear是提升动词，可以改为提升主语结构，比较分句可以改为另一种形式，即：

The difficulties are even greater than they appear (to be) at first sight. 比较对应项在从句作表语必须省略，they指代the difficulties 在从句作主语，由于they并不是比较对应项，此时they是不可以省略而保留谓语动词的。

我的看法和两位老师不同，问题句如果使用 appear 也是对的。对于 appear 的这种用法，现代英语中，很多人采用的方法是如果句子是单一相同对比项，对比项是复数，分句动词就用复数，而且分句对比项通常省略，反之，就动词用单数。也有人不使用这种方法，即使单一对比项为复数，分句动词仍然用单数，问题句就是一个例子。difficulties 为复数对比项，但than后动词用单数。当然如果用复数也对。

曹老师的例句：More people came to the party than (had been) invited. （出自《剑桥语法》）这里属于单一相同比较项people，从句省略的也是people，这种情况，虽然两个people数量不同，但分句people 包含在主句主语的people中，分句同类项要省略，避免逻辑上出现重叠。如果说受邀人来的比不受邀的人多，分句比较项一般不省略。More invited guests came to the party than those uninvited. 因为这属于不相同对比项，没有包含关系。

Alex 119老师提供的例句2，属于不同对比项，一个是person, 一个是they, 这时they不能省略。这种情形下，一般分句动词随对比项的单复数而变化。如： The person is less honest or moral than they appear to be. （复数）Alex119 老师的其他例句，我认为分句对比项省略也是对的。作者仅是选择了没省略而已。

另外，不是所有动词都可以这样用，《剑桥语法》曾列出了几十个包括appear在内的可以这样用的动词，但仅是常见的一部分，不是全部。

以上是我个人的理解，不一定对, 欢迎大家指正。我再补充几个外刊例句供比较参考。

appear 用单数：

1. The use of the land to provide "fuel" for horses or oxen also suggests that the contrast between the two countries may be greater than appears in the table.

2.The division between ellipticals and spirals is probably much less sharp than appears at first sight.

3.Deconstruction and empiricism, in fact, are closer to each other than appears on the surface.

appear 用复数：

1. The question of pegging to gold rather than dollars has two deeper implications for the system than appear at first sight.

2.They also average more populist attributes than appear on websites of small townships.

3. It is my impression that Yugoslav relations with the Soviet Union are actually much worse than appear on the surface.