连词解释
一.because,for,since,as,的区别
because语气强,表示客观必然原因：
He is absent¹, because he is ill. 因为生病，所以他没来。
比较:He is absent, for he is busy. （“生病”是“缺席”的必然原因，“忙”不是必然原因。）

for 语气轻,表示非客观必然的原因,是主观可改变结果的原因,甚至是猜测可能的原因:
He must be ill, for he is absent. “缺席”不一定是“生病”，只是交流猜测。
for 不能放句首,它是并列连词.

since,as 都是不讲自明的原因,是已知的原因.
Since I am a boy, let me carry the case.
As you don't feel well,you had better stay at home.

1.我不进去了,因为我爸爸在里面. I won't go in .for my father is there.
2.昨晚一定是下雨了,因为地很湿. It must have rained last night,for the ground is web².
3.地是湿的,因为昨晚下雨了. The ground is wet, because it rained last night.
4.因为不高兴,他不想出去了. He diden't wanted to go out,for he was unhappy.
5.既然你工作忙，你就不必跟我们一道去了。 Since/As you are busy, you had better not go with us.
6.他付给我比别人少的钱,因为我是个女的. He paid me less than the others, for I'm female³.
7.他付给我比别人少的钱,仅仅因为我是个女的. He paid me less than the others, only because I'm female.
8.既然大家都在这,我们开始开会吧. Since everybody is here,let's begin our meeting.

二.when,while,as引导时间状语时的异同.
when,表示时间点,时间段都可以.如状语是短暂动作时,多用when,是时间段时则三个连词都可以.
When/While/As I was walking in the street,I met a friend of mine.

只能用when 的句型:
1.放句中,主句是进行时,从句是短暂动作,表示这时突然
I was reading⁴ in the room, when a girl shouted for help.我正在房间看书，这时突然听到有一个女孩喊救命！

2.从句是短暂动作:When I got up, I heard the bell ring. 当我起床的时候，我听见铃响了。

只能用while的句型:
1.表示对立予盾的对比，这时while相当于but。
I am poor while you are rich.我很穷，你却很富有。
Tom is strong,while John is weak.  汤姆很强壮，而约翰很瘦弱。

2.表示“趁着还来得及的时候,赶快....
趁热打铁.Strike⁵ while it is hot. 
趁着老师还在教室,你赶快去问他吧. You should go to ask the teacher while he is still in.

只能用as的句型:
1.一边....一边...稳定的动作当从句,不稳定的动作当主句
他一边洗澡一边吹口哨. He whistled⁶ as he had a bath.
我一边看书一边听音乐. I listened to music as I read. 

2.正如....所知道,预料的一样.... as 在此是关系代词,不能用which代替.
正如众所周知的一样,地球是圆的. As everybody can see,the earth is round.
正如我们预料的那样，他失败了。He failed as we had expected.
正如我们所预料的一样,中国足球队赢了印尼. As we had expected,Chinese Football Team betean Indian 

3.随着时间的发展,某事变得......
As thirty years passed by,my mother's hair became gray. 三十年过去了，妈妈的头发成了银色。
As morden industry⁷ develops,more and more waste produces. 随着工业的发展，垃圾制造得越来越多。

三.as 与like的区别
1.表示象...一样时,as接从句,like 接短语
Do everything as I do. 象我一样做。
He is/looks like his mother. 他长得像他妈妈。

2.as当介词接短语时,表示作为...不是象...一样的意思.like 当动词时,是喜欢的意思,不要搞混为象...
We should study as Lenin studied. 我们应该像列宁那样学习。
As a League⁸ Member,I should take everything in the lead. 作为一名团员，我应该起带头作用。

三.untill,
I'll not go untill the bell rings.
unless 条件I'll not go unless you go.除非你走我才走。 
肯定句延续I waited untill he came 我一直等到他来。
He lived here until he was 90. 在90岁之前他一直住在这。
I didn't leave untill he came.直到他来我才走。
短暂not until,
I won't stay with you unless you drive the dog out.除非你把狗赶跑，否则我不会和你呆在一起。

四.and,but,however,yet,
顺趋势自然而然发展He studied hard and became a college student.

逆趋势转折He studied hard, but failed in the exam.

中间有逗号，语气轻用however He studied hard, however,he failed in the exam.
不能用but He studied hard, yet he failed in the exam. 
Althought he studied hard, yet he failed in the exam.（yet可以与although，though连用，but不能。

五.就近原则，对称原则，附加不理原则
谓语动词就最近距离主语原则：
Either he or his parents (is, are) wrong.
Either you or he (is, are) wrong.
(Is, Are) you or he wrong?

连词后成分对称原则：
Both ....and, neither....nor,  not only.....but also.......
She can (either sing, sing either)English songs or Chinese songs. 
She can (either sing, sing either)English songs or dance well.

with后名词附加不影响谓语原则
He as well as his wife and daughters (like, likes) music very much.
He with his sons (get, gets )up early every day.
I but you (are, am)wrong.