一、 非限制性定语从句通常不能用that引导。如：

　　1. I like the book, which was bought yesterday. 我喜欢这本书，这是昨天买的。

　　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. 我已经把理由告诉了他们，为此我没有去开会。

　　2. I had told them the reason why 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句首时，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 $20,000, with which his father was angry. 他花两万多美元买了这辆车，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. 我提前干完了我的活，随后我坐着看了一会儿报纸。

　　五、 在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，无论指人还是指物都不能省略。)