因为连词的种类非常多，有并列连词（连接简单句），有从属连词（引导复杂句），而从属连词引导的从句又可分为三类：名词性从句、定语从句和状语从句。尤其应注意的是有些连词可引导不止一种从句，具有多重功能，这往往是学习的难点，也是易混点。
难点回顾：
1 as可引导多种从句，如时间状语从句、原因状语从句、让步状语从句、方式状语从句、比较状语从句以及定语从句。
[误] Which you can see,he is always ready to help others.
[正] As you can see,he is always ready to help others.
[析]as引导非限制性定语从句，当"正像"讲。
[误] Do like I told you.
[正] Do as I told you.
[析]like是介词； as是连词，在这里作"按照"讲，引导方式状语从句。
[误] He was reading then he was walking.
[正] He was reading as he was walking.
[析]as强调两个动作同时进行，作"一边……一边……"讲。
[误] As he is young,he knows a lot.
[正] Young as he is,he knows a lot.
[析]as引导的让步状语从句应用倒装语序，应把表语提前。
2.that可引导多种从句，如定语从句、名词性从句、状语从句等。
[误] You don’t like him is none of my business.
[正] That you don’t like him is none of my business.
[析]that引导主语从句，本身无实际意义，但不能省略。
[误] The thief handed everything which he had stolen to the police.
[正] The thief handed everything that he had stolen to the police.
[析]everything是不定代词，因此后面的定语从句只能由that引导。
[误] I am happy as you passed the exam.
[正] I am happy that you passed the exam.
[析]that在形容词后面引导宾语从句，不能用as。
3.where可引导多种从句，如定语从句、状语从句以及名词性从句。
[误] Go to find your watch. It’s there where you left it.
[正] Go to find your watch. It’s where you left it.
[析]where引导表语从句，相当于at the place where。
[误] The place where there is water,there is life.
[正] Where there is water,there is life.
[析]where引导地点状语从句，可以放在主句的前面，也可以放在主句的后面。
[误] I can’t remember in which place I met him.
[正] I can’t remember where I met him.
[析]where引导宾语从句，不能用in which。
4.what可引导感叹句、特殊疑问句、名词性从句等。
[误] How an interesting story he told us!
[正] What an interesting story he told us!
[析]What an interesting story！=How interesting a story！
[误] I can’t remember the thing what he told me.
[正] I can’t remember what he told me.
[析]what引导宾语从句，相当于the thing that。
5 no matter+what/who...与whatever/whoever...的区别：前者只能引导让步状语从句，而后者既可引导让步状语从句，也可引导名词性从句。
[误] I will make friends with no matter who shares my interest.
[正] I will make friends with whoever shares my interest.
[析]引导名词性从句时只能用whoever。
[误] You must hand in no matter what you’ve found.
[正] You must hand in whatever you’ve found.
[析] 引导名词性从句时只能用whatever。
6.whether和if的区别：两者引导名词性从句时都是从属连词，作"是否"讲，本身不作成分，此时句子中往往出现表示"不肯定"意义的一些短语，如not sure/certain,not known/decided,...is still question,depend on等。
whether 和if 引导动词后的宾语从句时可互换，但下列情况一般只能用whether:
(1) 引导介词后的宾语从句；（2）引导主语从句； （3） 引导表语从句；（4）引导同位语从句；（5）后面出现or not。
[误] If you can pass the exam depends on how hard you work.
[正] Whether you can pass the exam depends on how hard you work.
[析]引导主语从句只能用whether。
7.while,when,as 引导时间状语从句时的区别：
while只能表示"在一段时间或过程中"，即只能表示时间的"一段"，后面一般用进行时或表示状态的结构；
when常与一般时态连用，用于表示一个动作与另一个动作在同一时间发生，或一个动作在另一动作所延续的时间范围内发生。可以表示一段时间，也可表示时间的"一点"；
When it is wet the buses are crowded.
天下雨时公共汽车就很拥挤。
When she pressde the button the lift stopped.
她按了按键，电梯停住了。
as引导时间状语从句时，表示两个动作同时进行，作"一边……一边……"讲。还可表示第二个动作发生在第一个动作结束之前。
[误] I picked up some French words as I was having a holiday in Paris.
[正] I picked up some French words while I was having a holiday in Paris.
[析]while强调在……过程中；as说明两个动作同时进行。
[误] We were having classes while someone knocked at the door.
[正] We were having classes when someone knocked at the door.
[析]when可表示时间的"一点"，而while不能。
8.because,as,since(now that）引导原因状语从句时的区别：
because引导原因状语从句时，表示直接的原因，可以回答why的提问；as只能是一种"附加的"理由，不能回答why的提问；since(now that）多表示双方都已很清楚的事实，作"既然"讲。
[误] Because we’ve finished most of the work,let’s have a rest.
[正] Since(Now that) we’ve finished most of the work,let’s have a rest.
[析]表示大家都清楚的理由用since(now that）。
9.however和as 引导让步状语从句时的区别：
however引导让步状语从句时，相当于no matter how，后面跟形容词或副词；as引导让步状语从句时，用倒装语序，即把作状语的副词或作表语的形容词或名词提前，而且如果作表语的是单数名词，前面不加冠词，也可以把谓语动词部分的实义动词提前。
[误] As hard he works,he can’t catch up with his classmates.
[正] However hard he works,he can’t catch up with his classmates.
[析]见上述说明。
[误] A model worker he is,he remains modest.
[正] Model worker as he is,he remains modest.
[析]as 引导让步状语从句，前面的单数名词前不加冠词。