一、非限制性定语从句不能用that引导。如：
1. I like the book, which was bought yesterday．我喜欢这本书，这是昨买的。
2. I like the book which/that was bought yesterday．我喜欢昨天买的那本书。
二、非限制性定语从句不能用why引导。要用for which代替why．如：
1. I had told them the reason, for which I didn’t attend the meeting．
我把理由告诉了他们，为此我不用去开会。
2. I had told them the reason why/for which I didn’t attend the meeting．
我告诉了他们我不去开会的理由。（限制性定语从句“the reason why…”是常见搭配。）
三、非限制性定语从句置于主句前时，不能用which引导；关系代词as引导非限制性定语从句位置比较灵活，可以放主句前，也可以放主句后。如：
1. As I expected, he didn’t believe me．正如我所预料的，他不相信我。
2. She heard a terrible noise, which brought her heart into her mouth．
她听到一个可怕的声音，这让她把心都提到嗓子眼了。
四、非限制性定语从句由“介词+关系代词”引导时，其中的关系代词不能用as，如：
1. He bought the car for more than $20000, with which his father was angry．
他花2万多美元买了这辆车，他父亲对此很生气。
2. I finished my work ahead of time, after which I sat some time reading the newspaper．
我提前完成了我的活，随后我坐着看了一会儿报纸。
五、非限制性定语从句中指人的关系代词作宾语时，只能用宾格whom, 不能用who替换，也不能省略。如：
1. Do you know Tom, whom we talked about? 你认识汤姆吗？我们谈到过他。
2. This book, which you can get at any bookshop, will give you all the information you need．
这本书会给你提供所有你需要的资料，它在任何一家书店都能买到。（非限制性定语从句中引导词在从句中作宾语，无论指人，指事物都不能省略）
