定语从句.
变主语从句 Who break the law will be punished.
区分时可以看从句是否可以删除,如果删除从句,句子还完整,那就是定语从句; 如果删除后不再是一个句子,就是主语从句,例如
Those who break the law will be punished -> Those will be punished
Who break the law will be punished -> will be punished.