句型16.

　　由as引导的非限定性的定语从句。如：

　　As we have known, he is a most good student.众所周知，他是个很好的学生。请比较：It is well-known that he is a most good student.(前一个是定语从句，而后者是个主语从句。)

　　句型17.

　　由which引导的非限定性的定语从句。如：

　　He is a professor, which I have been looking forward to becoming. 他是个教授，那是我一直盼望的职业。(因为先行词professor是表示职业的名词，因此引导词用which，而不用who。

　　句型18.

　　由where, when引导的定语从句(其中包括限定性的或非限定性的。)如：

　　This is the house where I used to come.请比较：This is the house which / that I used to come to.

　　This is the day when I joined the Party.请比较：This is the day which / that I joined the Party on.

　　(说明：关于that与which之间的区别，请看关系代词that和which的区别。)