1、当while, when, as引导时间状语从句时的区别：
(1) while引导的状语从句中动词必须是持续性。谓语动词多为进行时，或状态动词的一般时。while 的这些用法可用when代替。
E.g. Please keep quiet while (when) others are studying；
(2) when除可指一段时间外，还可用来指一点时间，等于 “at the time”，也就是说when引出的时间状语从句中的谓语动词可以是终止性的，也可以延续性的。因此主句和从句的谓语可以是一般时，进行时，或完成时。
E.g. When I went into the lab, the teacher was doing an experiment.（when不能换成while）He often makes mistakes when he is speaking English.（when可换成while）
(3) as常可与when，while通用，但强调“一边、一边”。E.g. As (when, while) I was walking down the street, I noticed a police car in front of number 37.
E.g. I’ll come when (if) I’m free.
2、till, until引导时间状语短语或状语从句，用于否定句时，结构为not …until (till)，主句谓语动词延续与非延续皆可，意为“直到…才…”。用于肯定句时，只与延续性动词连用，表示“到…为止”。E.g. They played volleyball until (till) it got dark./ They didn’t talk（延续性动词）until (till) the interpreter（译员）came./ He didn’t go to bed（非延续性动词）until (till) the his father came back.；until可以放在句首，till则不行，E.g. Until the last minute of the match we kept on playing./ Not until he finished his work did he go home.（倒装）；till, until只用于时间，以下句子是错误的：We walked till the edge of the forest.（要用as far as或to）。
语法易错题二、地点状语从句引导地点状语从句的连词是where 和wherever等。E.g. Sit wherever you like.Make a mark where you have a question.三、条件状语从句条件状语从句分真实性（有可能实现的事情）与非真实性（条件与事实相反或者在说话者看来不大可能实现的事情）条件句。引导条件状语从句的词（组）主要有if, unless, so (as) long as, on condition that, so (as) far as, if only ( = if )。注意：条件从句中的if 不能用whether替换。 E.g. If he is not in the office, he must be out for lunch.You may borrow the book so long as you keep it clean.So far as I know（据我所知）, he will be away for three months.You can go swimming on condition that ( = if ) you don’t go too far away from the river bank.If he had come a few minutes earlier, he could have seen her.四、原因状语从句because, since, as引导原因状语时注意使用上的区别：1.如果原因构成句子的最主要部分,一般用because ,因此because引导的从句往往放在句末。用why提问的句子，一定用because回答。E.g. He had to stay at home yesterday because he was ill.；2.如原因已为人们所知，或不如句子的其他部分重要，就用as，或 since。since比as更正式些。as和since引导的从句一般放在句子的开头。E.g. As you are tired, you had better rest./ Since everyone is here, now let’s begin. 五、结果状语从句结果状语从句是表示事态结果的从句，通常主句是原因，从句是结果。由so that （从句谓语一般没有情态动词）, so … that, such … that等引导。 E.g. She was ill, so that she didn’t attend the meeting.He was so excited that he could not say a word. She is such a good teacher that everyone admires her.六、目的状语从句引导目的状语从句最常用的词（组）是so, so that（从句谓语常有情态动词）, in order that, in case（以防，以免）等。E.g. Speak clearly, so that they may understand you.She has bought the book in order that she could follow the TV lessons.He left early in case he should miss the train. 七、让步状语从句让步状语从句可由although, though, as, even if (though), however, whatever, whether … or, no matter who (when, what, …) 等引导。注意：as引导的让步状语从句一般是倒装的。E.g. Though he is a child, he knows a lot.Child as he is, he knows a lot. Whatever ( = No matter what ) you say, I’ll never change my mind.八、比较状语从句比较状语从句常用than, so (as) … as, the more … the more等引导。E.g. I have made a lot more mistakes than you have.He smokes cigarettes as expensive as he can afford.The busier he is, the happier he feels.九、方式状语从句方式状语从句常由as, as if (though), the way, rather than等引导。E.g. You must do the exercise as I show you.He acted as if nothing had happened.