1）先行词和关系词合二为一的情况，例如：
Whoever spits in public will be punished here.(Whoever 可以用 anyone who 代替)
The parents will use what they have to send their son to technical school.(what 可以用all that代替)
注意all that 和what的用法区别：what 实际上只引导名词性从句，相当于all that ，例如：
All that I know is that he has made up his mind to heave the country for a new place.= What I know is that he has...
注意that 和what 的用法区别：当that引导定语从句时
，通常用作关系代词，而引导名词性从句时，是个不充当任何成分的连接词。宾语从句和表语从句中的that常可省略。What只能引导名词性从句，用作连接代词，作从句的具体成分，且不能省略。例如：
I think (that) you will like the stamps.（宾语从句）
What we need is more practice.（主语从句）
2）以the way为先行词的定语从句通常由in which， that引导，而且that通常可以省略，例如：
The way in which/(that)/he answered the question was surprising.
3) but有时也可以做关系词引导定语从句，例如：
There are very few but understand his idea. ( but= who don’t ）
4）误认为关系代词whose只用于修饰人。
whose用作疑问代词时，主要用于指人；但用作关系代词时，它既可指人也可指物。例如：
It was an island, whose name I have forgotten. 它是一座岛，名字我忘了。
The factory, whose workers are all women, is closed during the holidays. 这家工厂工人都是妇女，在假期中工厂关门了。
5）误认为逗号后一定是非限制性定语从句。
不要一看见逗号，就以为一定要选which而不选that，有时它根本就不是定语从句。例如：
(1) If a book is in English, _______ means slow progress for you.
A. as B. which C. what D. that
(2) When I say two hours, _______ includes time for eating.
A. as B. which C. what D. that
分析：以上两题均应选that，而不能选which，因为它们根本不是定语从句。之所以选that，是因为句中已有if和when引导的状语从句，逗号后为主句，that为主句主语。
6）混淆用关系代词与关系副词。
先行词是表示时间、地点、原因的名词时，不一定用关系副词。还要看关系词在定语从句中是用作什么成分，若是用作状语，则用关系副词，若不是用作状语(如用作主语、宾语等)要用关系代词。例如：
This is the factory where I want to work. 这就是我想工作的工厂。(work为不及物动词where在从句中用作状语）
This is the factory that I want to visit. 这就是我想参观的工厂。(visit为及物动词，that用作visit的宾语）
The reason why he can’t come is that he is ill. 他没来的是因为他病了。(come为不及物动词，why在定语从句中用作状语）
The reason that he put forth is very important. 他提出的理由很重要。(put forth为及物动词，that在定语从句中用作其宾语）
7）混淆定语从句和并列句。例如：
（1）He has two children, and both of _________ are abroad.
A. them B. which C. whom D. who
（2）He has two children, both of _________ are abroad.
A. them B. which C. whom D. who
分析:第1题选A，第2题选C。由于第1题中用了并列连词and，从而使整个句子为并列句，and后应是一个独立的简单句，所以选A不选C；第2题没有并列连词and，both of whom are abroad为非限制性定语从句。另外，请比较下面一题:
He has two children, both of _________ being abroad.
A.them B. which C. whom D. who
分析:此题答案为A，其中的both of them being abroad为独立主格结构，用作状语.
8)混淆定语从句和表语(从句).例如:
(1) Your coat is still _____ you left it.
A. whereB. thereC. there where D. the place where
(2) Is this school _____ your father worked in ten years ago ?
A. whereB. whatC. thatD. the one
分析:第1题选A，where引导的是表语从句(=在…的地方)，而不是定语从句。第二题选D，this school为句子主语，the one 为表语，your father worked in…为定语从句。不要误认为this是句子主语，否则school前应加冠词the。比较：
Is this the school _____ your father worked in ten years ago?
A. where B. what C. that D. the one (选C)
Is this the school _____ your father worked ten years ago?
A. where B. what C. that D. the one (选A)
9）混淆定语从句和状语从句。例如：
(1) I went upstairs ______I heard some crying.
A. where B. when C. that D. which
分析：选B，而不是A，when的意思是“当…的时候”，引导的是时间状语从句。(upstairs为副词，不宜用作先行词)
(2) He is _____ a clever boy _____ we all like.
A. such, that B. such, as C. so, that D. so, as
分析：选B，而不是A。such … that …(如此…以致…)用于引导结果状语从句，其中的that不充当句子成分，而such…as…中的as为关系代词，用以引导定语从句并在定语从句充当句子成分。上句中第二空之所以选as，是因为它用作动词like宾语。
10）混淆定语从句和同位语从句。
二者的区别：
a)定语从句修饰先行词，它和先行词是修饰关系；同位语从句说明先行词的具体内容，是补充说明的关系. The plane that has just taken off is for London. 定语从句 The fact that he has been dead is clear. 同位语从句 b)定语从句由关系代词或者关系副词引导，关系词在句中充当成分，有时可以省略；同位语从句主要由that引导，在句中一般不做成分；句子也可以由when， where， how， why， whether， what等词引导，充当成分. The problem that we are facing now is how we can collect so much money. 定语从句 The problem how we can collect so much money is difficult to solve. 同位语从句 c)同位语从句和先行词一般可以用be动词发展成一个完整的句子，而定语从句不可以. The idea that we could ask the teacher for advice is wonderful. 同位语从句 The idea was that we could ask the teacher for advice. The fact that the earth moves around the earth is known to all.同位语从句 The fact is that the earth moves around the earth.
11）混淆定语从句和强调句。例如： --Where did you find the professor who made the speech yesterday? --It was in the hall ______ the students often have a meeting. A.where B.which C.that D.when 分析：很可能误选C，认为这是一个强调句，强调地点状语 in the hall。假若选C，即有 It was in the hall that the students often have a meeting,该句的意思是“学生们通常是在大厅开会”，单独看这一句，无论是其意思还是其语法均未错，但若将其与上文联系起来看，则不通，因为上文的意思是“你是在哪儿找到昨天作报告的那位教授的?”假若将答句改为 It was in the hall that I found the professor，则完全可以。其实，此题的最佳答案是A，where the students often have a meeting 为定语从句，用以修饰其前的名词 the hall，句意为“是在学生们经常开会的那个大厅(找到教授的)”，这样语意就通顺了。
