2.两种定语从句的引导词不同。非限制性定语从句的引导词不能替代或省略。
限制性定语从句
非限制性定语从句
①关系代词可用who(m)/whose/which/that
①关系代词可用who(m)/whose/which/(that×)
②关系副词可用when/where/why
②关系副词可用when/where/(why×)
③引导词有时可以代替。如，
who代替whom，that代替who(m)/which。
③引导词不能替代。（为什么that不能引导非限制性定语从句呢？因为that本质是替代词。）
④引导词有时可以省略。
④引导词不能省略。
例句（that不能引导非限制性定语从句）
●He failed again, which/(that×) was out of my expectation.
●This note was left by Tom, who(that×) was here just now.
●First came Mary, who(m)/(that×) few of us had expected.
二.分割性定语从句
1.什么是分隔性定语从句？即，先行词+…+定语从句。
一般地，定语从句紧跟在先行词后边，即，先行词+定语从句。有时，为了避免头重脚轻，把定语从句移到句尾，而导致定语从句不是紧跟在先行词后边，即，先行词+…+定语从句，这种情况叫“分隔性定语从句”。
被哪些结构分隔呢？我们在第一章讲过“四结构”：动词短语，介词短语，非谓语动词短语，从句。定语从句被分隔，其实就是各种结构之间调整了顺序而已。如，
●I was the only person in my office that was invited.（被介词短语分隔）
我是我们办公室里唯一被邀请的人。
●The days will come soon when people have enough to eat and wear.（被谓语分隔）
人们的丰衣足食的日子很快就会到来。
●There are no places left on the earth that the foot of man has not trodden.（被非谓语动词分隔）