780.The mistakes children make in learning to speak tell linguists more about how they learn language than the correct forms they use.
(A) how they learn language than
(B) how one learns language than
(C) how children learn language than do
(D) learning language than
(E) their language learning than do
the key:C

than后面的do是做什么用的？我看大全注释里面说do代替tell，不懂怎么个代替法。

The mistakes [children make in learning to speak] tell linguists more about (C) how children learn language than do the correct forms they use.
句子主干是The mistakes tell linguists more about ... than do the correct forms they use.

所以do就是替代tell的简洁用法。

这样看清楚了么？