when before 等的区别

when
〔引导时间状语从句〕具体地说，它有以下几种含义：
1．表明一个动作发生的时候，另一个动作正在进行之中。

例如：
When the teacher came into the classroom ，we were reading English．当老师走进教室时，我们正在读英语。

2．表明当某一动作正在进行时，另一动作发生了。

如：
When Kate was flying a kite，she found a wallet lying on the ground．凯特正在放风筝时，她发现地上有个钱包。

〔特别提示〕当when从句放在主句之后时，它更强调某一动作的突然性，此时的when相当于and then，and suddenly，just at that time。

如；
1．We were watching TV when the telephone rang．（＝We were watching TV and suddenly the telephone rang．）我们正在看电视，（突然）电话铃响了。

试比较：When we were watching TV，the telephone rang．我们正在看电视时，电话铃响了。

2．The boys were playing football when it began to rain．男孩子们正在踢球时，（不料）开始下雨了。

3．I was taking a walk when I came across Lily．我正在散步时，（没想到）遇见了莉莉。

特别是主句中有just，或when从句中有suddenly时，这种突然性更为明显、强烈。

Until 直到….为止, not… until直到…才
肯定形式表示的意思是"做某事直至某时"，动词必须是延续性的。

否定形式表达的意思是"直至某时才做某事"。

动词为延续性或非延续性都可以。

肯定句：
I slept until midnight. 我一直睡到半夜时醒了。

Wait till I call you. 等着我叫你。

（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.
她直到6点才到。

否定句可用另外两种句式表示。

(1)Not until …在句首，主句用倒装。

Not until the early years of the 19th century did man know what heat is.
直到19 世纪初，人类才知道热能是什么。

Not until I began to work did I realize how much time I had wasted.
直到我开始工作，我才认识到了我已蹉跎了几多岁月。

(2) It is not until… that…这是一个强调结构。

如本题。

before引导的从句不再用否定式的谓语，并且当before引导的从句位于主句之后，有时译成“就，才”。

还要注意主句和从句之间的时间关系。

当主句用将来时，从句总是用现在时；如果before引导的从句谓语用的是过去时，则主句动词多用过去完成时，这样以便体现动作发生的先后。

It will be four days before they come back. 他们要过四天才能回来。

Einstein almost knocked me down before he saw me.爱因斯坦几乎把我撞倒才看到我。

since
一、若主句的谓语动词是现在时态，从句的谓语动词是非持续性动词，则从句的动词只能用一般过去式：
I．It is(or has been)two months since he fell ill．(不能用has or had
fallen i11)(自)他生病(以来)已有两个月了。

2．It has been(or is)about ten minutes since the film began．(不能用has
or had begun)(自)电影开演(到现在)大约已有10分钟了。

二、若主句和从句的谓语动词都是过去时态，从句的谓语动词又是非持
续性动词，主句和从句的谓语动词时态搭配则有四种情况：
I．It was two months since he fell ill．(自)他生病(以后)到那时已有
两个月了。

(以下三句的译文跟该句相同)
2．It had been two months since he had fallen i11．
3．It had been two months since he fell i11．
4．It was two months since he had fallen ill．
三、若主句的谓语动词是现在时态，从句的谓语动词是持续性动词的过
去时形式，since从句表示的时间应从该从句动词的终点算起：
I．It is(or has been)two months since he was ill(was ill=got well)(自)
他病愈(以来)已有两个月了。

2．It is(or has been)two months since he lived here．(1ived here=moved
away)(自)他搬走(至今)已有两个月了。

四、若主句的谓语动词为现在时态，从句的谓语动词是持续性动词的现
在完成时形式，则since从句所表示的时问，要从该持续动词的起点算起：
1.It's(or has been)two months since he has beenillhas been ill=fell
ill)(自)他生病(以来)已有两个月了。

2．It's(or has been)two months since he has lived here．(has lived
here=moved here)(自)他搬到这儿居住(以来)已有两个月了。

五、若主句的谓语动词是过去时态，从句的谓语动词用持续性动词的过去式跟用其过去完成时形式意义相反：
I．It had been(or was)two months since he had lived here．（had lived here=moved here)(自)他搬来居住(到那时)已有两个月了。

2．It had been(or was)two months since he lived here．(1ived here=moved away)(自)他搬走(那时)已有两个月了。