主语补足语
出现主语补足语的场合大多数是在有不定式被动语态的场合，被动语态后面的补足语称作主语补足语。通常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。
可以做主语补足语的有名词，形容词，介词短语，感官动词+分词/不定式，what引导的名词性从句，as+名词/形容词/分词等。
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：
① He is often heard reading English.
② The professor was seen making an experiment in the chemistry lab.
③ The glass was found broken.
④ The classroom was found crowded with people.
4.感官动词see, watch, hear, notice, feel, make等后面接带to的不定式用作主语补足语。例如：
① He was seen to come upstairs.
② Ice is known to be in a solid state.
③ The spy was ordered to be hanged.
5. 介词短语用作主语补足语。例如：
① The books in the study must be kept in good order.
② He was found in good health.
③ English is considered of great importance for us.
6. as后面接名词、形容词、分词等用作主语补足语。例如：
① English is taken as a useful means for research work.
② The news is considered as true.
③ The stool is usually thought as having four legs
④ The vase is thought as broken.
7. 由what引导的名词性从句用作主语补足语。例如：
The boy has been made what he is.
三、 主语补足语的判别
1.看句中的动词是不是可接复合宾语，而且是不是被动语态，与此同时还要看其后部分的逻辑主语是不是句子的主语。
2. 另一种最简单的方法是：如果还不能看出来就可以把全句改成主动语态，加上一个主语we或people等。改成主动语态后，看后面是不是变成了“宾语+宾语补足语”了，这样我们就可以判别原句后面是不是主语补足语。例如：
被动句：She was found reading in the library. (主语补足语)
主动句：We found her reading in the library. (宾语+宾语补足语)