当前面句子中的动词是行为动词时省略号处用副词；是系动词时用形容词
因为副词修饰限定动词如 run fast,study hard,listen carefully,dance well
系动词后接形容词作表语 be important,be popular,look happy,be careful
We should study as hard as possible
We should be as careful as possible 所以我们应说
When we speak to people ,we should be as polite as possible