whom 为什么在表语从句中不能作主语

whom 不管在什么从句里都不能作主语，只能作宾语。能作主语的是who，而且who也能作宾语。

whom可用于表语从句，主语从句，宾语从句和定语从句。例句如下：

1、Whom you will trust is always your parents.(主语从句)

2、The question is whom you can turn to for help when in trouble.(表语从句)

3、We don't know whom the manager will choose to be his assitant.(宾语从句)

4、The teacher introduced a star to us whom the teenagers like most.(定语从句)

这些句子中的whom都可以换成who，并且定语从句中的还可以省略。