一、混淆定语从句与并列句
有的句子结构相似，相差的可能只是一个并列连词，同学们若忽略此并列连词，则可能混淆定语从句与并列句。请看下面两道试题：
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：
He has two children, both of ______ being abroad.
A. them B. which C. whom D. who
【分析】此题选A，尽管句中没有并列连词，但由于逗号后并不是一个完整的句子(因句中的 being 为非谓语动词)。
二、混淆定语从句与表语(从句)
一般说来，若是定语从句，其前必有先行词；若是表语(从句)，则其前必有连系动词。但问题是，有时由于句子结构比较特殊，定语从句与其修饰的先行词可能被“分离”，而连系动词与其后的表语也可能被“分离”，这就需要同学们认真分析，理顺句子结构(最好能还原句子)，从而分清是定语从句还是表语(从句)。请看下面两道试题：
1. Your coat is still ______ you left it.
A. where B. there C. there where D. the place where
【分析】答案选A，where引导的是表语从句(=在……的地方)，而不是定语从句。
2. Is this school ______ your father worked in ten years ago?
A. where B. what C. that D. the one
【分析】答案选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。
三、混淆定语从句与状语从句
有些试题，从表面看它是定语从句，而实际上它是状语从句；而有些试题则可能完全相反，即从表面看它是状语从句，而实际上是定语从句。请看下面两道试题：
1. I went upstairs ______I heard someone crying.
A. where B. when C. that D. which
【分析】答案是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】答案选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的宾语。
四、混淆关系代词与关系副词
有的同学一看到先行词是表示时间、地点、原因的名词，就认为一定要用关系副词，其实也不一定。在此情况下，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)则不能用关系副词，而用关系代词。比较：
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在定语从句中用作状语)
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)
五、因逗号误判which
有的同学一看见逗号，就以为一定要选which而不选that，但问题是有时根本就不是定语从句。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。
六、混淆which与whose
两者在定语从句中都可用作定语，区别是：whose 的意思相当于 one’s，而 which 的意思则相当于 that 或 this。比较：
1. This is Mary, whose father we met last week.
2. Call again at 11, by which time the meeting should be over.
【分析】第1句用whose，whose father相当于 and her father；第2句用which，by which time 相当于 and by that time。
七、弄错定语从句中谓语动词的数
原则上说，定语从句中谓语动词的单复数应与所修饰先行词的单复数保持一致，否则就会出错。如：
谁那样说都是在说谎。
误：Anyone who say that is lying.
正：Anyone who says that is lying.
不算生病的，我们一共20人。
误：There are 20 of us, not counting those who is sick.
正：There are 20 of us, not counting those who are sick.
八、受汉语影响导致定语从句用词累赘或成分残缺
一方面，引导定语从句的关系代词与定语从句修饰的先行词表示同样的概念，因此，千万不可在定语从句中误加与先行词同义的代词；另一方面，也不能受汉语习惯的影响，省去定语从句中不该省略的必要成分。如：
他是一个人人都敬重的人。
误：He is a man that everyone respects him.
正：He is a man that everyone respects.
刚才给我们讲故事的那个人很有趣。
误：The man told us a story just now is very interesting.
正：The man who told us a story just now is very interesting.