系动词后面的肯定是表语，那样请问主系表结构，是否表示系动词必须要跟在主语后面？
下面两个句子的keep和appear是系动词吧？但不是紧跟主语后面，这是主系表结构吗？是补充说明主语吗？
I asked them to keep quiet.
I tried to appear resigned to defeat.
I asked them to keep quiet.
I tried to appear resigned to defeat.
这是两个定义。黑体部分叫做动词不定式，而这个动词不定式是由系表结构组成。不定式本身的结构也是各式各样。比如，有些不定式是不及物动词；有些不定式是不及物动词加状语；有些不定式是及物动词加宾语；有些不定式是及物动词加复合宾语，等等，不一而足。比如：
I asked him to help.
I asked him to help as soon as possible.
I asked him to help us.
I asked him to help us clean the window.
