求解一句话，考比较结构的！

charles1992

Because natural gas is composed mostly of methane, a simple hydrocarbon, vehicles powered by natural gas emit less of certain pollutants than those burning gasoline or diesel fuel.

为什么those前面不加do呢？

In an assessment of the problem faced by rural migrant workers, the question of whether they are better off materially than the urban working poor is irrelevant.

为什么the urban前面不加are呢？

例如：The guiding principles of the tax plan released by the Treasury Department could have even greater significance for the economy than do the particulars of the plan.

困惑：不明白什么时候加，什么时候不加？麻烦大家给个详细的解答呀

ChinaHawk

我觉得比较的双方是主系表结果的时候如第二个例子，是可以省略系动词的，不知道是不是这样？

with二

我觉得这三个句子都是可加可不加，你可以翻看一下Aeo版主以前的帖子，涉及到比较的部分。

michellehoS

最后一个不补出来就有歧义了
可以理解成 economy和 particulars of the plan 相比较 这样就不对了
应该是 The guiding principles of the tax plan 和the particulars of the plan 相比

没有歧义的时候就不加 有歧义的时候才加 不知道我说没说清楚 呃