宾语补足语是对宾语的进一步补充说明。在逻辑上有主谓关系。
同位语的前面有名词，同位语从句所表示的内容与其前面的名词所表示的内容是完全一致的。例句：
1、Mary's parents often ask her to study.
不定式to study就是宾语her的补足语。
2、I saw Tom coming into the room.
现在分词coming就是宾语Tom的补足语。
3、I made a promise that if anyony set me free， I would make him very rich.
if引导的从句与名词promise是同一内容。
4、We heard the news that Mary won the first prize for physics competition.
that引导的从句与名词news是同一内容。
