He hopes the book ismore universalthanitappearson the cover.（原句）
He hopes the book ismore universalthanappearson the cover.（省略it）
Tom singsbetterthan he dances
是不是也能改为Tom sings better than dances?
有关参考：
http://ask.yygrammar.com/q-25138.html
He hopes the book ismore universalthanitappearson the cover.（正）
He hopes the book ismore universalthanappearson the cover.（误）
缘由是：假如把than 看作appears 的主语，那样，appeaers 就缺少表语。因此，需要在than 后添加主语it；由于than 不可以既当主语，又当表语。
1、在than 从句中，只有在谓语完整的状况下，than 才能充当从句的主语。
比如：
He never says more than is necessary.
他从不多说没必要的话。（他从不说废话）
He often eats more than is good for his health.
他常常吃的饭量多于对身体有益的量。
The family spent more than is prudent or necessary.
该家庭烧钱过多，超越了必要的限度。
It is not wise to be wiser than is necessary.
聪明过了头，就是不聪明。
The man drank a little more than was good for him.
那个人酒喝得过了量。
Always do more than is required of you.
任何事永远要超需要完成。
He offered more information than is needed.
他提供了多余的信息。
2、假如谓语部分缺少成分，than 就充当缺少的成分，这个时候，than 句需要另有主语。
比如：
The reality may be far brighter than it looks.
实质状况或许要比看起来的要光明得多。
All in all, hes more learned than he was ten years ago.
总的说来，他比10年前更有学问了。
Tom writes English better than John does（writes）.
汤姆英语的书写比约翰好。
The question is more complicated than it appears on the surface.
这个问题比表面看起来要复杂得多。
3、假如比较的是完全不一样的内容，than 就是从属连词，后面的句子是完整的。
比如：
Tom writes English better than he speaks it.
汤姆的英文写的比说的好。
We had no sooner left the village than it began to rain.
大家刚离开村子，天就下起雨来了。
（字面意思是：大家离开村庄的时间不早于天下雨。但，由于主句用了过去完成时，仍然表明离开早于下雨。）
than 有哪些用途，除去表示“比较”意义外，就是充当和补足从句中缺少的成分。
