The mistakes children make in learning to speak tell linguist more about how they learn language than the correct forms they use.
The correct answer is: how children learn language than do
这里的do是否可以省去，我觉得do 加在这里反而不对，因为do 是指代tell，是不是加了do 反而使得形式不对称。
The average-income and unemployment figure，这里的figure指数据时是不是不加s表复数概念？
1.要加do，而且形式很对称。
简化为A tell linguist more than do B.
2.引用princess的贴子
看了Kilroy的问题后恍然大悟，B中对称的是average-income和 unemployment figure，而不是我原来以为的average-income figure和 unemployment figure。可能是因为average-income这个指标本身就应该是一个数字
先将句子补充完整:
The mistakes children make in learning to speak tell linguist more about how children learn language than the correct forms they use (tell linguist how chidren learn language.)
故可以用do代替括号内的动词tell.
do可以倒装也可以顺序.
let me try to answer your confusion, hopefully helpful.
if no "do", "the correct forms they use" sounds like parallel with "how clause",the 'do' here is added to avoid this kind of misleading.There are a lot of ways of omiting in parallel structure, the basic rule is no misleading in meaning.
