LiBin does his homewore as carefully as Lin Dong.为什么这个地方用carefully?用careful不行吗？
为什么？谢谢啦。

LiBin does his homework as carefully as Lin Dong.
这句话里面，carefully在句中的作用是什么？修饰动词does!只有副词能修饰动词，所以就用carefully.
一般的，当句子的主谓结构是主系表结构，就用adj,如：
This film is as interesting as that one
She looks as pretty as before.她看起来和以前一样漂亮
This rope is as long as that one.这条绳子和那条一样长。
This pizza tastes as good as that one.这个披萨跟那个一样好吃。
一般的，当句子的主谓结构是主+实意动词，就用adv,如：
LiBin does his homework as carefully as Lin Dong.
Please answer my question as soon as possible.
请尽快回答我的问题。
It took us as long as three years to carry out the plan.
我们花了长达三年时间才完成这项计划。

这个提法也许不是完全严谨，但实用上是很管用的。