He sat here,silent.该句为什么用形容词作状语,而不用副词silently?

在你的句子里,silent做主语的补足语.可以改写成：He sat there and was silent.主语补足语的目的是用来补充说明主语.== He was silent when he sat there.
又如：
She was hungry when she went to bed=== She went to bed hungry.(hungry是主语的补足语).