非限制性定语从句：是对前面先行词起进一步补充,解释说明的作用
He has a son ,who is a university student
who is a university student 为非限制性定语从句：对前面先行词son 起进一步的补充,解释说明的作用
主语从句是由一个连接词+句子充当主句的主语
例如：When he will come back is unknown
When he will come back 为主句的主语
非限制性定语从句：是对前面先行词起进一步补充,解释说明的作用
He has a son ,who is a university student
who is a university student 为非限制性定语从句：对前面先行词son 起进一步的补充,解释说明的作用
主语从句是由一个连接词+句子充当主句的主语
例如：When he will come back is unknown
When he will come back 为主句的主语