状语从句按其意义和作用可分为:a)时间状语从句b)地点状语从句c)原因状语从句d)目的状语从句e)结果状语从句f)条件状语从句g)方式状语从句h)比较状语从句i)让步状语从句2.时间状语从句引导时间状语从句的从属连词有：when, while, as, whenever, before, after, since, till, until, as soon as, once, hardly…when, no sooner…than等引导。
如：When he was still a young man, he was forced to leave his homeland for political reasons.注意：a)when, as, whilei.when即可引导持续性动作，又可引导短暂性动作。
它可以表示主句的动作和从句的动作同时发生，或从句的动作发生在主句动作之前。
如：When I was a boy, I used to go to the seashore on Sundays.（同时）When the lesson was over, we began our writing.（从句动作在前）ii.as引导持续性动作，侧重主句和从句的动作同时发生。
He hurried home, looking behind as he went.I saw your sister as she was getting on the bus yesterday.He sang as he walked.iii.While：指的是“在某一时间里”，“在……期间”，从句里的动作必须是持续性的，它也强调主句和从句动作的同时发生，往往侧重主句和从句动作的对比。
如：While he was in London, he studied music.While we were watching TV, he was writing a composition.while也可做并列连词，表示对照的含义，意思为“而”。
如：He is tall while his brother is short.当when, as, while表示“在…一段时间里”，主从句的动作同时发生时，可以换用。
如：While (When或As) we were discussing, Mr. Smith came in.b)如果when和before引导的从句位于主句之后，有时不能译成“当…时候” 和“在…之前”，而要译成“就”、“才”、“这时”等。
如：The struggle lasted four years before the North won in the end.He almost knocked me down before he saw me.She had not been married many weeks when the man saw her and was struck by her beauty.I was walking in the street when I saw him.c)by the time, each time, every time, immediately, the moment, the instant, the minute, soon after, shortly after都可以作为连词，引导时间状语。
如：By the time he was fourteen, he had taught himself advanced mathematics. Each time he came, he would call on me.You must show him in immediately he comes.I recognized him the moment I saw him.d)by the time 的用法i.by the time+现在时，主句用将来完成时．例如：By the time he arrives, we will already have left California.ii.by the time+一般过去时，主句用过去完成时．例如：By the time he arrived, we had already left California.e)till和untili.如果主句谓语动词是持续动词，通常用肯定式的主句，表示“直到…为止”。
如：I worked till (until) he came back.ii.如果主句谓语动词是瞬间动词，则用否定式的主句表示“直到……才”。
如：I didn’t go to bed until (till) he came back.iii.放在句首表示强调时一般用until。
如：Until he returns, nothing can be done.注意：Not until his mother called did he get up.(不倒装为He didn’t get up until his mother called.)It was not until his mother called that he got up.(not until的强调结构为it is/was not until…that…)f)before 的句型：i.It won’t be long before sb. does sth. 不用多久某人就会……ii.It will be long before sb. does 要过很久某人才会……iii.It was long before sb. did sth. 过很久某人才做……iv.It will be + 一段时间+ before sb. does 某人要过多长时间才做v.It was +一段时间+ before sb. did sth. 某人过了多长时间才做g)用no sooner …than 和hardly…when 引导的从句表示“刚……就……”，主句中的动词一般用过去完成时，从句用过去时若把no sooner, hardly 提到句首，主句倒装例如：He had no sooner got to the lab than he set out to do the experiment.(该句倒装为：No sooner had he got to the lab than he set out to do the experiment.)h)时间状语从句中谓语动词不能用任何一种将来时，只能用现在时和过去时表示将来时。
3.地点状语从句:通常由连词where和wherever等引导。
如：Go back where you came from.Wherever you go, you must write to your parents.Where there is a will, there is a way.Where there is too much, the poison and waste may do great harm to the things around us.4.原因状语从句:通常由连词：because, as, since, now that引导。
a)区别是:because：表示“因为”，直接而明确的原因和理由，语气最强，why提问的句子，一般都用because回答。
He didn’t come because he was ill.since：表示“既然”，语气比because弱。
Since you are here, you must do it.as：表示“因为”语气比because轻，引导从句可放主句前也可放主句后。
You needn’t go with me, as you are busy.As I was afraid, I hid myself.now that意思与since相似，表示“既然”。
Now that you are in High School, you will probably spend more in reading. Now that all the guests have arrived, let’s have our dinner.b)注意：i.在强调句中强调原因状语从句，只能用because引导，不可用as或since，如：It was because he was ill that he didn’t go with us.ii. because可以引导表语从句,而as, since不可以，这时主语一般都是it, this, thatIt’s because he is too lazy.for:也表示“因为”，但是并连词，它连接的不是状语从句，语气比较强。
For引导从句不说明主句发生的直接原因，只是提供一些补充说明，且不位于句首．It must be morning,for the birds are singing.seeing that (既然) 和in that (因为)Seeing that the weather is bad, we’ll stay at home.I like the city, but I like the country better in that I have more friends in the country.5.目的状语从句:通常由that, so that, in order that, so…that, lest, in case（以防、免得）等引导：He got up early in order that he could catch the early bus.She married him so that she might tend and comfort him.I explained again and again in case he should misunderstand me.目的状语从句中谓语动词常含有may (might), can (could) should, will等情态动词。
lest (以免，免得), for fear thatI hid the book lest (=for fear that) he should see it.6.结果状语从句:由that, so that, so…that, such…that引导：It was very cold, so that the river froze.The book is so written that it gives a quite wrong idea of the facts.There were so many people (=such a lot of people) in the room that we could not get in.He made such an excellent speech that every one admired him.7.条件状语从句:由if, unless, as long as (=so long as) (只要), in case (that) (如果,万一)等引导：If plastics and rubber are burned, they give off poisonous gases.Unless you work hard, you will fail,You can go out, as long as (so long as ) you promise to be back before eleven. In case I forget, please remind me about that.注意：条件状语从句中的谓语动词的时态不可用将来时，只能用现在时态或过去时态表示将来时。