主语补足语是一种句子成分，顾名思义，是补充主语的成分。以be supposed to do为例，其中的to do是主语补足语。
思路是这样的：
be supposed to do显然源自suppose sb to do。suppose sb to do中suppose是动词，sb是宾语，to do的逻辑主语是sb，所以to do是宾语补足语。当suppose sb to do转变为被动形式的sb be supposed to do，sb成了主语，to do的逻辑主语仍是sb，所以to do相应地就成了主语补足语。