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)