我们看一下夸克的例句：

They say he plays truant, which he doesn't. [l]

He walks for an hour each morning, which would bore me.  [2]

以上两个句子中，which的先行词无疑不包括句子的主语，而只是黑体部分, 即先行词为谓语动词+宾语或状语，即谓语部分。二个句子不同之处在于，第一句中的which表面上看等于play truant, 实际是doesn't后省略了do，which意为it。而第二句则which直接等于it，it指代walking for an hour each morning.

Things then improved, which surprises me.  [3]

Colin married my sister and I married his brother, which makes Colin an! me double in-laws. [4]

以上二个句子中，which的先行词为前面整个主句的内容。

我们再看剑桥英语语法的例句：

He told me to design it myself,  which I simply can't.

They advised me to call the police , which I did immediately.

这二个例句显然which的先行词为主句中的动词不定式。

改为并列句结构，则上述二句分别为：

He told me to design it myself, but I simply can't do it.

They advised me to call the police, and I did it immediately.

以上二句删去并列连词，用which替换it，就得到这种句子关系分句。

所以，which引导非限制性定语从句，其先行词既可以是前面整个主句的内容，也可以是主句的谓语动词，以及非谓语动词部分。夸克把这种which从句称为句子关系分句sentential relative clause，剑桥英语语法把这种which分句称为补充语分句supplementary clause