1．二者差异比较
限制定语从句紧跟先行词，同先行词这间一般不加逗号，仅修饰先行词，可以由关系代词．关系副词或that来引导。非限制性定语从句仅作补充或说明，用逗号与主句隔开，既可修饰先行词，又可修饰整个主句，不可用that引导。
2. that可引导定语从句表示时间．地点或原因
That有时可以代替关系副词 when, where 或者why引导定语从句表示时间．地点或原因，在 that引导的这种定语从句中，that也可以省去。
1．关系副词也可以引导定语从句
关系副词在从句中分别表示时间．地点或原因。关系副词when在从句中充当时间状语，where 充当地点状语，why充当原因状语。
3．“介词+关系代词“是一个普遍使用的结构
(1) “介词+关系代词“可以引导限制性定语从句，也可以引导非限制性定语从句。“介词+关系代词“结构中的介词可以是 in, on, about, from, for, with, to at, of, without 等，关系代词只可用whom或 which,不可用 that 。
(2) from where为“介词+关系副词“结构，但也可以引导定语从句。例如：
We stood at the top of the hill, from where we can see the town..
(3) 像listen to, look at, depend on, pay attention to, take care of等固定短语动词，在定语从句中一般不宜将介词与动词分开。例如：
This is the boy whom she has taken care of.
2．关系代词的用法
(1) 如果先行词是all, much, anything, something, nothing, everything, little, none等不定代词，关系代词一般只用that，不用which。例如：
All the people that are present burst into tears.
(2) 如果先等词被形容词最高级以及first, last, any, only, few, mush, no, some, very等词修饰，关系代词常用that,不用which, who，或whom。例如：
(3) 非限制性定语从句中，不能用关系代词that,作宾语用的关系代词也不能省略。例如：
There are about seven million people taking part in the election, most of whom、are well educated.
(4) which还有一种特殊用法，它可以引导从句修饰前面的整个主句，代替主句所表示的整体概念或部分概念。在这种从句中，which可以作主语，也可以作宾语或表语，多数情况下意思是与and this 相似，并可以指人。例如：
He succeeded in the competition, which made his parents very happy.
(5) that可指人或物，在从句中作表语，(指人作主语时多用who)仅用于限制性定语从句中。
(6) which可作表语，既可指人，以可指物。指人时，一般指从事某种职业或是有种特征．品性或才能的人。Which引导的定语从句可以限制性的，也可以是非限制性的。
(7) 如果作先作词的集体名词着眼于集体的整体，关系代词用which;若是指集体中的各个成员，则用who。
(8) 先行词有两个，一个指人，一个指物，关系代词应该用that。例如：
The boy and the dog that are in the picture are very lovely.
(9) 如果先行词是anyone, anybody, everyone, everybody, someone, somebody,关系代词应该用 who 或whom，不用 which。例如：
Is there anyone here who will go with you?
1. 关系代词用来指代先行词是人或物的名词或代词
例1：This is the detective who came from London.
例2：The book which I am reading is written by Tomas Hardy.
例3：The desk whose leg is broken is very old.
例4：This is the room that Shakespeare was born in.
带to 的情态动词有四个：ought to, have to, used to, be to, 如加上have got to ,(=must), be able to为六个,它们的疑问，否定形式应予以注意：
Do they have to pay their bill before the end of the month?
She didn‘t use to play tennis before she was fourteen.
You ought not to have told her all about it.
Ought he to see a heart specialist at once.?
例：Tom ought not to ___ me your secret, but he meant no harm.
A.have told B.tell
C.be telling D. having told
第4讲：定语从句
用来说明主句中某一名词或代词(有时也可说明整个主句或主句中一部分)而起定语作用的句子叫作定语从句。
1) may(might) have + done sth, can (could) have + done sth表示过去，推测过去时间里可能发生的事情。
Philip may (might) have been hurt seriously in the car accident.Philip can (could) have been hurt seriously in the car accident.
2) must have +done sth，对过去时间里可能发生的事情的推测，语气较强，具有"肯定"，"谅必"的意思。-
--Linda has gone to work, but her bicycle is still here.
---She must have gone by bus.
3) ought to have done sth, should have done sth.本应该做某事，而事实上并没有做。否定句表示"不该做某事而做了"。You ought to (should) have been more careful in this experiment. He ought not to have thrown the old clothes away.(事实上已扔了。)
ought to 在语气上比should 要强。
4) needn't have done sth 本没必要做某事
I dressed very warmly for the trip, but I needn't have done so. The weather was hot.
5) would like to have done sth 本打算做某事
I would like to have read the article, but I was very busy then.
can, could, may, might, must 皆可表示推测，其用法如下：
1)情态动词+动词原形。
表示对现在或将来的情况的推测，此时动词通常为系动词。I don't know where she is, she may be in Wuhan.
2)情态动词+动词现在进行时。
表示对现在或将来正在进行的情况进行推测。
At this moment, our teacher must be correcting our exam papers.
这时，我们老师想必在批改试卷
3)情态动词+动词完成时。
表示对过去情况的推测。
We would have finished this work by the end of next December.明年十二月底前我们很可能已完成这项工作了。The road is wet. It must have rained last night.地是湿的，昨天晚上一定下雨了。
4)情态动词+动词的现在完成进行时。
表示对过去正在发生事情的推测。
Your mother must have been looking for you.你妈妈一定一直在找你。
5)推测的否定形式，疑问形式用can't, couldn't表示。 Mike can't have found his car, for he came to work by bus this morning. 迈克一定还没有找回他的车，因为早上他是坐公共汽车来上班的。
注意：could, might表示推测时不表示时态，其推测的程度不如can, may。
must 用于肯定句
can 用于否定句或疑问句
May 用于肯定句或否定句
should 用于肯定句