四六级英语学习，语法是英语学习的基础，也是的基础，掌握好英语语法无论是对还是以后的英语学习都是有很大帮助的。下面频道为大家整理了常考知识讲解，一起来学习一下吧!英语常考知识点讲解：定语从句引导词。
　　that, who, whom: 非性定语从句, 如果修饰人, 一般用who, 有时用that (作主语时用who较多). 如果关系代词在从句中作宾语, 就应当用宾格 whom 或that, 但在大多数情况下都可以省略掉, 在口语中可用who代替whom. Here is the man (whom) you’ve been looking for. He is a man (that) you can safely depend on. The people (who/that) you were talking to were Swedes. There are some people here who I want you to meet. 但在介词后只能用whom: This is the man to whom I referred.
　　但在口语中一般都把介词放到句子后面去, 这时可用that, 但省略时更多一些. Have you met the person about whom he was speaking? Have you met the person (that) he was speaking about The girl to whom I spoke is my cousin. The girl (who/that) I spoke to is my cousin.
　　性定语从句如果修饰“物”, 用关系代词that的时候较多, 也有时用which.. 当这个代词在从句中是用作宾语时, 在绝大多数情况下都是省略的, 特别是口语中(尤其是当被修饰的词是all, everything等词时): Have you everything you need? (Is there) anything I can do for you? All you have to do is to press the button.
　　在介词后只能用which, 在口语中一般都把介词放到从句后部去, 这时可以用that, 但省略的时候更多一些: The tool with which he is working is calLED a wrench. The tool (that) he is working with is called a wrench. This is the question about which we’ve had so much discussion. This is the question (that) we’ve had so much discussion about.
　　定语从句一般是修饰名词或代词的, 但间或也可以修饰整个句子a), 或是句子的一部分 b), 引导词用which: They have invited me to visit their country, which is very kind of them. The activity was postponed, which was exactly what we wanted When deeply absorbed in work, which he often was, he would forget all about eating and sleeping. She was very patient towards the children, which her husband seldom was.
　　whose: 在表示“...的”这个概念时, 可用所有格 whose; whose 用于指物, 有时可与of which交替使用, 通常的词序是名词词组 of which: Is there anyone in your class whose family is in the northeast? We had a meeting whose purpose was completely unclear. (…the purpose of which was…) He’s written a book the name of which I’ve completely forgotten. (…whose name I’ve…) of which前的名词词组也可以由some, any, none, all, both, several, enough, many, most, few以及基数词担任; 这些词也能用在 of whom之前. The buses, most of which were already full, were surrounded by an angry crowd. It’s a family of eight children, all of whom are studying music.
　　关系副词 when, where, why: 它们的含义相当于 at which, in which, for which, 因此它们之间有交替使用的可能. The day when he was born… on which he was born… which he was born on… The office where he works… at which he works… which he works at…
　　有时可用that替代关系副词, 在口语中that 可省略. Every time (that) the telephone rings, he gets nervous. This was the first time I had serious trouble with my boss. Do you know anywhere (that) I can get a drink? This is the place (where) we met yesterday. That is the reason (why) he did it.
　　在the way 后也可用that 替代in which, 在口语中that也可省略. This is the way (that/in which) he did it. That’s the way I look at it. 如果定语从句中谓语为 there is, 作主语的关系代词也常可省掉: I must make full use of the time there is left to me and do as much as I can for the people. This is the fastest train (that) there is to Nanjing.
　　定语从句的简化: 定语从句与不定式结构, -ing分词结构, -ed分词结构以及无动词分句等有着转换关系. He was the only one to realize the danger (= who realized the danger). The woman driving the car (= who was driving the car) indicated that she was going to turn left. The man injured by the bullet (= who was injured by the bullet) was taken to hospital. All the women present (= who were present) looked up in alarm.