关系代词who.which.that什么时候可以省略？

Question

如题

Answer 1

如果修饰事物，用关系代词which 和that，作宾语时可省略。作介词宾语时，如果介词位于关系代词之前，则用which (不能省) ，而不用that（关系代词that既能代人，又能代物）。whose也可用来指物（此时可用of which来代替），在定语从句中作定语。
如：
①Don’t buy tomatoes that/ which are green.
不要买颜色还是青的西红柿。
( that/ which 在定语从句中作主语，不能省)
②The fish (that/ which) they sell are not fresh.
他们卖的鱼不新鲜。
（ that/ which 在定语从句中作宾语，可以省）
③The classroom whose door is broken will soon be repaired.
= The classroom the door of which is broken will soon be repaired.
门坏了的那个教室将很快修理。
④The tool with which he is working is called a saw.
=The tool (that/ which) he is working is called a saw.
他干活用的那个工具叫叫锯。
（介词with位于关系代词之前，须用which，不能省，也不能用that）。

谢谢～