主补即主语补足语即被动语态中的宾语补足语。
eg.They caught boy stealing.
被动语态 The boy was caught stealing.
stealing 即为主语补足语
被动语态后的主语补足语
对于主语补足语，语法家们各有不同的看法。有的把连系动词后面的部分称作主语补足语；有的又把被动语态后的补足语称作表语，与被动语态一起称作复合谓语。笔者通过教学体会到，还是把被动语态后面的补足语称作主语补足语好些。这是因为学生很容易接受“宾语补足语”，而如果把带有宾语补足语的句子变成被动句，原来补充说明宾语的部分就变成补充说明主语了。
一、被动语态的主语补足语与主动语态的宾语补足语是密切相关的。例如：
(1) I saw him playing basketball yesterday.
(2) He was seen playing basketball yesterday.
句（1）中的含义不是我看见他，而是我看见他正在打篮球。playing basketball是宾语him的补足语。所以叫宾语补足语。
句（2）中的含义也不是他被看，而是别人看见他正在打篮球。这里的playing basketball是主语he的补足语，故称作主语补足语。主语和主语补足语一起称作复合主语。所以含有主语补足语的句子一般是被动语态，谓语动词是可以接复合宾语（宾语+宾语补足语）的及物动词。句首的主语就是主语补足语的逻辑主语。
二、主语补足语形式种种
1. 动词elect, call, name, make, find, leave等后面常接名词用作主语补足语。例如：
① The dog is called Karl.
② Coal is sometimes called stored-up sunlight.
③ He was found the right man for the job.
2. 动词keep, make, paint, cut, burn, beat, wash, find, consider, wipe等后面常接形容词用作主语补足语。例如：
① The door was painted white.
② The old man was found weak.
③ The classroom is always kept clean every day.
3. 动词see, watch, hear, feel, listen to, look at, imagine等后面接分词用作主语补足语。例如：