主句是全句的主體，通常可以獨立存在；從句則是壹個句子成分，不能獨立存在。
從句不能單獨成句，但它也有主語部分和謂語部分，就像壹個句子壹樣。所不同在於，從句須由壹個關聯詞（connective）引導。
復合句（The Complex Sentence）：句子中有壹個或壹個以上的從句，叫做復合句。復合句可分為：
1).定語從句（The Attributive Clause）；
2).狀語從句（The Adverbial Clause）；
3).名詞性從句（The Noun Clause）
壹、 定語從句
· 定語從句的定義
定語從句在句子中作定語，用來修飾壹個名詞、名詞詞組或者代詞。
· 先行詞和引導詞
被修飾的名詞、名詞詞組或代詞叫做先行詞；
在先行詞和定語從句之間起連接作用的詞叫做引導詞。
引導詞分為“關系代詞”和“關系副詞”。
· 關系代詞和關系副詞
關系代詞有：who, whom, whose, that, which, as。
關系副詞有：when, where, why。 註意：關系副詞裏面沒有how。
如果要修飾方式，用that或in which引導，或者不用引導詞。
I don’t like the way (that, in which) he eyed me.
我不喜歡他看我的那個樣子。
· 關系代詞：who
關系動詞who在從句中主要作主語，在非正式語體裏who還可以作從句中的賓語。
He is the man who wants to see you.
He is the man who I saw in the park yesterday.
· 關系代詞：whom
He is the man （whom） I saw in the park yesterday.
（whom在從句中作賓語）
· 關系代詞：whose
whose 用來指人或物，（只能用作定語, 若指物，它還可以同of which互換）。
They rushed over to help the man whose car had broken down.
Please pass me the book whose cover is green. （of which the cover/the cover of which）
· 關系代詞：which（1）
which指物，在從句中作主語或賓語，作賓語時在非正式語體中可以省略。
They needed a plant which didn't need as much water as rice.
The farm (which) we visited yesterday is located in the suburb of Beijing.
· 關系代詞：which（2）
當在which和that面前進行選擇的情況下，壹般情況下要選which：
1.在非限制性定語從句中通常用which作引導詞，而不能用that做非限制性定語從句的引導詞。
2. 修飾整個主句。
I never met Julia again after that, which was a pity.
3. 修飾謂語部分。
He can swim in the river, which I cannot.
4. 介詞 + which
They are all questions to which there are no answers.
· 關系代詞：that（1）
that多用來指物，有時也可以用來指人；在從句中作主語或賓語。指物的時候多用that，也可用which。
It’s a question that (which) needs careful consideration.
（指物，作主語。）
Who is the man that is reading a magazine under the tree?
（指人，作主語。）
The girl (that) we saw yesterday is Tom’s sister.
（指人，作賓語，可省略。）
· 關系代詞：that（2）
在以下的情況中，只能用that作引導詞，而不能用which作引導詞。
1. 先行詞為all , everything, anything, nothing, little等不定代詞時。
All (that) she lacked was training.
2. 先行詞被形容詞最高級修飾時
This is the best film that I have ever seen.
3. 先行詞被序數詞和the last修飾時
4. 先行詞中既有人又有物時
They talked about the teachers and the schools that they had visited.
5. 主句是含有who或which的特殊疑問句，為了避免重復時
二、 狀語從句
1 地點狀語從句
地點狀語從句通常由where, wherever 引導。
Where I live there are plenty of trees.
Wherever I am I will be thinking of you.
2 方式狀語從句
方式狀語從句通常由as, (just) as…so…, as if, as though引導。
1） as, (just) as…so…引導的方式狀語從句通常位於主句後，但在(just) as…so…結構中位於句首，這時as從句帶有比喻的含義，意思是"正如…"，"就像"，多用於正式文體，例如：
Always do to the others as you would be done by.
妳希望人家怎樣待妳，妳就要怎樣待人。
As water is to fish, so air is to man.
我們離不開空氣，猶如魚兒離不開水。
Just as we sweep our rooms, so we should sweep backward ideas from our minds.
正如打掃房屋壹樣，我們也要掃除我們頭腦中落後的東西。
2） as if, as though
兩者的意義和用法相同，引出的狀語從句謂語多用虛擬語氣，表示與事實相反，有時也用陳述語氣，表示所說情況是事實或實現的可能性較大。漢譯常作"仿佛……似的"，"好像……似的"，例如：
They completely ignore these facts as if (as though) they never existed.
他們完全忽略了這些事實，就仿佛它不存在似的。（與事實相反，謂語用虛擬語氣。）
He looks as if (as though) he had been hit by lighting.
他那樣子就像被雷擊了似的。（與事實相反，謂語用虛擬語氣。）
It looks as if the weather may pick up very soon.
看來天氣很快就會好起來。（實現的可能性較大，謂語用陳述語氣。）
說明：as if / as though也可以引導壹個分詞短語、不定式短語或無動詞短語，例如：
He stared at me as if seeing me for first time.
他目不轉睛地看著我，就像第壹次看見我似的。
He cleared his throat as if to say something.
他清了清嗓子，像要說什麽似的。
3 原因狀語從句
比較：because, since, as和for
1） because語勢最強，用來說明人所不知的原因，回答why提出的問題。當原因是顯而易見的或已為人們所知，就用as或 since。
I didn't go, because I was afraid.
Since /As the weather is so bad, we have to delay our journey.
2） 由because引導的從句如果放在句末，且前面有逗 號，則可以用for來代替。但如果不是說明直接原因，而是多種情況加以推斷，就只能用for。
He is absent today, because / for he is ill.
He must be ill, for he is absent today.
4 目的狀語從句
表示目的狀語的從句可以由that, so that, in order that, in case等詞引導，例如：
You must speak louder so that /in order that you can be heard by all.
Better take more clothes in case the weather is cold.
5 結果狀語從句
結果狀語從句常由so… that 或 such…that引導，掌握這兩個句型，首先要了解so 和 such與其後的詞的搭配規律。
比較：so和 such
其規律由so與such的不同詞性決定。such 是形容詞，修飾名詞或名詞詞組，so 是副詞，只能修飾形容詞或副詞。 so 還可與表示數量的形容詞many, few, much, little連用，形成固定搭配。
so foolish such a fool
so nice a flower such a nice flower
so many / few flowers such nice flowers
so much / little money. such rapid progress
so many people such a lot of people
（ so many 已成固定搭配，a lot of 雖相當於 many，但 a lot of 為名詞性的，只能用such搭配。）
so…that與such…that之間的轉換既為 so與such之間的轉換。
The boy is so young that he can't go to school.
He is such a young boy that he can't go to school
6 條件狀語從句
連接詞主要有 if, unless, as/so long as, on condition that 等。.
if 引導的條件句有真實條件句和非真實條件句兩種。非真實條件句已在虛擬語氣中闡述。
unless = if not.
Let's go out for a walk unless you are too tired.
If you are not too tied, let's go out for a walk.
7 讓步狀語從句
though, although
註意： 當有though, although時，後面的從句不能有but，但是 though 和yet可連用
Although it's raining, they are still working in the field.
3) ever if, even though. 即使
We'll make a trip even though the weather is bad.
4) whether…or- 不管……都
Whether you believe it or not, it is true.
5) "no matter +疑問詞" 或"疑問詞+後綴ever"
No matter what happened, he would not mind.
Whatever happened, he would not mind.
替換：no matter what = whatever
no matter who = whoever
no matter when = whenever
no matter where = wherever
no matter which = whichever
no matter how = however
註意：no matter 不能引導主語從句和賓語從句。
（錯）No matter what you say is of no use now.
（對）Whatever you say is of no use now.
妳現在說什麽也沒用了。(Whatever you say是主語從句)
（錯）Prisoners have to eat no matter what they're given,
（對）Prisoners have to eat whatever they're given. 囚犯們只能給什麽吃什麽。
8 比較while, when, as
1）as, when 引導短暫性動作的動詞。
Just as / Just when / When I stopped my car, a man came up to me.
2）當從句的動作發生於主句動作之前，只能用when 引導這個從句，不可用as 或 while。
When you have finished your work, you may have a rest.
3）從句表示"隨時間推移"連詞能用as，不用when 或while。
As the day went on, the weather got worse.
日子壹天天過去，天氣越變越壞。
9 比較until和till
此兩個連詞意義相同。肯定形式表示的意思是"做某事直至某時"，動詞必須是延續性的。否定形式表達的意思是"直至某時才做某事"。動詞為延續性或非延續性都可 以。 正確使用這兩個連詞的關鍵之壹就在於判斷句中的動詞該用肯定式還是否定式。
肯定句：
I slept until midnight. 我壹直睡到半夜時醒了。
Wait till I call you. 等著我叫妳。
（在肯定句中可用before代替 Let's get in the wheat before the sun sets.）
否定句：
She didn't arrive until 6 o'clock.
Don't get off the bus until it has stopped.
1)Until可用於句首，而till通常不用於句首。
Until you told me, I had heard nothing of what happened.
直到妳告訴我以前，出了什麽事我壹點也不知道。
2)Until when 疑問句中，until要放在句首。
---Until when are you staying? 妳呆到什麽時候？
--- Until next Monday. 呆到下周壹。
註意：否定句可用另外兩種句式表示。
(1)Not until …在句首，主句用倒裝。
Not until the early years of the 19th century did man know what heat is.
直到19 世紀初，人類才知道熱能是什麽。
Not until I began to work did I realize how much time I had wasted.
直到我開始工作，我才認識到了我已蹉跎了幾多歲月。
(2) It is not until… that…
10 表示"壹…就…"的結構
hardly/scarcely…when/before, no sooner…than 和as soon as都可以表示"壹…就…"的意思，例：
I had hardly / scarcely got home when it began to rain.
I had no sooner got home than it began to rain.
As soon as I got home, it began to rain.
註意：如果hardly, scarcely 或no sooner置於句首，句子必須用倒裝結構：
Hardly / Scarcely had I got home when it began to rain.
No sooner had I got home than it began to rain.
三、 名詞性從句
1）.賓語從句（The Object Clause）；
· 賓語從句
在句子中起賓語作用的從句叫做賓語從句。賓語從句的引導詞和主語從句的引導詞相同。賓語從句可以作及物動詞的賓語，作短語動詞的賓語，介詞的賓語。
· 賓語從句：及物動詞
Everybody knows that money doesn't grow on trees.
· 賓語從句：短語動詞
Please go and find out when the train will arrive.
· 賓語從句：介詞的賓語
I am interested in what she is doing.
· 賓語從句：否定的轉移
I don't suppose you're used to this diet.
我想妳不習慣這種飲食。
I don't believe she'll arrive before 8.
我相信她8點之前不會到。
2）.表語從句（The Predicative Clause）；
· 表語從句
在句子中起表語作用的從句叫做表語從句。表語從句的引導詞和主語從句的引導詞相同。
What the police want to know is when you entered the room.
This is what we should do.
That's why I want you to work there.
as if, as though, because也可用來引導表語從句。
She seems as if she had done a great thing.
It is because you eat too much.
· 虛擬語氣：表語從句
主語是idea, notion, proposal, suggestion, request等名詞時,作表語從句的動詞為原形動詞或should+原形動詞。
My suggestion is that we (should) go and help him.
Our only request is that this should be settled as soon as possible.
3）.同位語從句（The Appositive Clause）。
· 同位從句壹般跟在某些名詞如answer(答案)，hope(希望)，fact(事實)，belief (信仰)，news(消息)，idea(主意；觀念)，promise(承諾)，information(信息)，conclusion(結論)，order(命令)， suggestion(建議)，problem(問題)，thought (想法)等後面。例如：
I had no idea that you were here.
She told us her hope that she would become a pianist.
He made a promise that he would never come late.
· 同位語從句：whether
whether可以引導同位語從句，而定語從句不能用whether作為引導詞。
He hasn’t made the decision whether he will go there.
I have small doubt whether he is suitable for the job.
· 同位語從句：what
what可以引導同位語從句，而定語從句不能用what作為引導詞
I have no idea what he is doing now.
· 同位語從句：how
how可以引導同位語從句，而定語從句不能用how作為引導詞
It’s a question how he did it.
· 同位語從句和定語從句的區別（1）
同位語從句和先行詞是同等的關系；而定語從句是用來修飾先行詞，是從屬的關系。
· 同位語從句和定語從句的區別（2）
that在同位語從句中沒有詞義，不充當句子成分；而在定語從句中充當主語、賓語等句子成分。
· 同位語從句和定語從句的區別（3）
whether, what, how可以用來引導同位語從句；而它們不能用來引導定語從句。