英语从句里It is/was...后面用since或before究竟有什么区别？不太清楚！麻烦说得详细点。
选B 吧，since 是自什么以来，自什么以后！before 在什么以前，还没来得及做什么，前面的东作就已经发生！在本句中用before 应该用完成式吧或过去式也可用！应该翻译成你认为他到那儿已经多长时间了？
before [bi'fɔ:]
a. 以前,在...以前
ad. 以前
prep.&conj. 在...以前
before [biˈfɔ:]
prep.在…前 conj.在…以前 ad.以前
before
Wiki
a.
1. 以前,在...以前
ad.
1. 以前
Come at two o'clock, not before.
两点钟来,不必再早了。
He was never late before.
过去他从不迟到。
Have you two met before?
你们两人以前见过面吗?
conj.
1. 在...以前
Get out before I call the police.
滚开,否则我叫警察。
I'd die before I'd tell.
我宁死也决不会说出来。
Check it carefully before you hand it in.
交来之前仔细核对一下。
prep.
1. [时间]...以前
the night before last
前晚
She said she had never spoken before an audience.
她说她从未在大庭广众讲演过。
Pride comes before a fall.
骄必败。
the day beforeyesterday
前天
before nine o'clock
九点以前
Only two days remained before the examination.
考前只剩下两天了。
Your name comes before mine.
你的名字在我之前。
before Christ
[缩写 B.C.]公元前
We will put the matter before the committee.
我将把此事提交委员会考虑。
2. [位置]在...的前面; 当...面前
He sat just before me.
他就坐在我的前面。
3. [位次]先于; 高于
A general comes before a colonel.
将官比校官高。
4. [选择]与其...宁愿
Death before dishonor!
宁死不辱。
She desired no one for husband before him.
她非他不嫁。
I've seen that film before.
我以前看过那部影片.
He will die before he submits.
他宁死不屈。
He always put the interests of others before his own.
他总把他人的利益放在第一位。
I have seen that film before.
那部电影我以前看过。
B comes before C in the alphabet.
B在字母表里排在C之前.
He made a statement before the House of Commons.
他在下议院当众发表了一个声明。
It will be a long time before we finish this dictionary.
我们需要很长的时间才能把这部词典编译完。
We had dinner after/before they arrived.
他们到达之后[之前]我们吃的饭.
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since
Wiki
ad.
1. 后来
ad.(常用于现在完成时或过去完成时)
1. 此后;从那时到现在
She left school three years ago and has worked as a nurse ever since.
她三年前毕业,从那时起便一直当护士。
conj.
1. 既然,因为
Since it is so hot, let's go swimming.
既然天气这么热,我们去游泳吧。
It's been ten years since they married.
他们结婚到现在已经十年了。
She has been living a hard life since her husband died.
她自从丈夫死后,一直过着艰苦的生活。
He must have taken the book since it isn't here.
他一定是把书拿走了,因为书已不在这里了。
prep.
1. 自... 以来, ...以后
since liberation
解放以来
I haven't heard from him since last year.
我自去年以来未曾收到过他的信。
We've been friends ever since we met at school.
自从我们在学校认识之后，我们就一直是好朋友。
She moved to London last May and has since got a job on a newspaper.
她去年五月?铰锥兀?酸嵋恢痹诒ㄉ绻ぷ鳌?
Where have you been since I last saw you?
自从我上次见到你以后, 你到哪儿去了?
It was the first time I'd won since I'd learnt to play chess.
自从我学会下国际象棋以来，这是我第一次赢。
Since you can't answer the question, perhaps we'd better ask someone else.
既然你无法回答这个问题，我们最好再问问别人。
I haven't eaten since breakfast.
我吃了早饭后到现在还什麽都没吃呢.
He had spoken to her only once since the party.
自从那次聚会以来, 他只跟她说过一次话.
How long is it since we visited your mother?
自从我们看望你母亲以来到现在有多长时间了？
He has studied very hard since he came to our school.自从他来到我们学校，他学习就非常努力。
We have been missing them since they left here自从他们离开这里，我们就一直很想念他们。
这是最常见的一种用法，此处不再多述。
二、若since引导的状语从句的谓语动词是持续性动词或表示状态的动词的过去时时，则从句表示的时间是“从那持续动作或状态结束时算起”。如：
I haven’t heard any noise since I slept. Sleep 为持续性动词，sleep的动作结束时，即“醒来”时，这句应译为“我醒后还未听到任何声音”。
三、若since引导的状语从句的谓语为持续性动词的现在完成时，则表示动作和状态延续到现在（说话时刻），其表示的时间的起点应从动作发生之时算起。如：
I haven’t heard from him since he has lived there。
这里has lived表示动作的持续性，时间的起点应从：“开始居住”时算起。因此此句可理解为“自从他（开始）住在那儿起，我就一直没收到他的来信”。
至于It is +一段时间+since…的句型与It has been +一段时间+since…意思相同。前者是规范说法，后者是口语说法。
应注意的是，在这种结构中，since引导的从句绝对不能用否定式的谓语动词来表示否定意义，下面两句都错了：
It is a long time since he didn’t study English.
It’s two months since you didn’t come to see me.
正确的说法为：
It’s a long time since he studied English 他好久没学英语了。
You haven’t come to see me for two months. 你有两个月没来看我了。
since的用法
1) since +过去一个时间点 (如具体的年、月、日期、钟点、1980, last month, half past six)。
I have been here since 1989.
2) since +一段时间+ ago
I have been here since five months ago.
3) since +从句
Great changes have taken place since you left.
Considerable time has elapsed since we have been here.
4) It is +一段时间+ since从句
It is two years since I became a postgraduate student.
since的用法主要有两点：一是表示“自从……以来”，二是表示“既然”之意。但如果只了解词的含义，而不了解其用法，在使用时就会造成错误。例如：I have not heard from him since I lived in Shanghai.
不少人会望文生义，将该句译为：“自从我住在上海以来，就再也没有收到他的信了。”而实际上这个句子的译文并非如此，那么正确的含义是什么呢？
下面谈谈since作连词引导时间状语从句的用法。
一、若since引导的状语从句的谓语动词是终止性的过去时，则从句表示的时间是“从动作开始的那一时刻起”。如：
He has studied very hard since he came to our school.自从他来到我们学校，他学习就非常努力。
We have been missing them since they left here自从他们离开这里，我们就一直很想念他们。
这是最常见的一种用法，此处不再多述。
二、若since引导的状语从句的谓语动词是持续性动词或表示状态的动词的过去时时，则从句表示的时间是“从那持续动作或状态结束时算起”。如：
I haven’t heard any noise since I slept. Sleep 为持续性动词，sleep的动作结束时，即“醒来”时，这句应译为“我醒后还未听到任何声音”。
John is now with his parents in New York, it is already three years since he was a teacher约翰现在和父母一起住在纽约，他不当教师已经三年了。
本文开始出现的那个句子就可以译为“自从我离开上海，就再也没有收到他的信。”这种译法常被人们误用，我们不妨再多举几个例子看看：
How long is it since you lived in Shanghai?你离开（没住在）上海多久了？
It is half a month since he was a League member.他退团（不当团员）半个月了。
It’s been quite some time since I was last in London我离开伦敦已很长时间了。
比较：He has written to me frequently since I was ill.自从我病愈以来，他屡次给我写信。（从句谓语为状态动词）
He has written to me frequently since I fell ill.自从我生病以来，他就屡次给我写信。（从句谓语为终止性动词）
三、若since引导的状语从句的谓语为持续性动词的现在完成时，则表示动作和状态延续到现在（说话时刻），其表示的时间的起点应从动作发生之时算起。如：
I haven’t heard from him since he has lived there。
这里has lived表示动作的持续性，时间的起点应从：“开始居住”时算起。因此此句可理解为“自从他（开始）住在那儿起，我就一直没收到他的来信”。
Since we have owned a car, we have gone camping every year。自从我们自己有了汽车，我们每年都去野营。
试比较：1. Since I’ve been at this school, we have had three headmasters.自从我到这所学校，我们已换了三个校长了。
Since I was at this school, they have had three headmasters.自从我离开这所学校，他们已换了三个校长了。
2.He has never been to see me since I have been ill. 自从我生病以来，他从未来看过我。
He has never been to see me since I was ill. 自从我病愈以来，他从未来看过我。
由此可见，since引导的从句持续动词的一般过去时和现在完成时所表示的意思恰好相反。换言之，前者以肯定的语法形式反映着否定的逻辑内容，而后者的形式与内容是一致的。
至于It is +一段时间+since…的句型与It has been +一段时间+since…意思相同。前者是规范说法，后者是口语说法。
应注意的是，在这种结构中，since引导的从句绝对不能用否定式的谓语动词来表示否定意义，下面两句都错了：
It is a long time since he didn’t study English.
It’s two months since you didn’t come to see me.
正确的说法为：
It’s a long time since he studied English 他好久没学英语了。
You haven’t come to see me for two months. 你有两个月没来看我了。
一、若since引导的状语从句的谓语动词是终止性的过去时，则从句表示的时间是“从动作开始的那一时刻起”。如：
He has studied very hard since he came to our school.自从他来到我们学校，他学习就非常努力。
We have been missing them since they left here自从他们离开这里，我们就一直很想念他们。
这是最常见的一种用法，此处不再多述。
二、若since引导的状语从句的谓语动词是持续性动词或表示状态的动词的过去时时，则从句表示的时间是“从那持续动作或状态结束时算起”。如：
I haven’t heard any noise since I slept. Sleep 为持续性动词，sleep的动作结束时，即“醒来”时，这句应译为“我醒后还未听到任何声音”。
三、若since引导的状语从句的谓语为持续性动词的现在完成时，则表示动作和状态延续到现在（说话时刻），其表示的时间的起点应从动作发生之时算起。如：
I haven’t heard from him since he has lived there。
这里has lived表示动作的持续性，时间的起点应从：“开始居住”时算起。因此此句可理解为“自从他（开始）住在那儿起，我就一直没收到他的来信”。
至于It is +一段时间+since…的句型与It has been +一段时间+since…意思相同。前者是规范说法，后者是口语说法。
应注意的是，在这种结构中，since引导的从句绝对不能用否定式的谓语动词来表示否定
如果你要表达“还未……就，不到……就，……才……，没来得及”的意思时就要用before。before后面的从句可以用现在式也可以过去式。
since从句的谓语动词一般是是非延续性动词。（翻译的时候要否定的。）
在it is+时间+since这个句型中时间的计算一律从since从句的动作完成或状态结束时算起。（有点文绉绉的额。）举例子吧，就是：
it is three years since i lived here（live是延续性动词）我住在这里有2年了。
it is 3 years since i smoked a cigarette（=since i stopped smoking a cigaret啊te）我不吸烟有3年了。（smoke是短暂性动词）
如果是“我开始吸烟有3年了，就是“it is 3 years since i began to smoke。”
以下是一些结构：
1、it is/was +时间+since从句，从句用一般过去式。
表示“自从……以来已经……（多久了）”
e.g. it is/has been two years since i began to learn English。
2、it wasn't long before……不久就……
it won't be long before ……过了多久才
it was +时间（2 years）+ before……两年以后就……
it was not 2 years before……不到两年就
it won't be +时间（2 days）+before……不到2天就……
it will be 2 days before……过了两天才……
从句如果用since引导，表示自从…句子用现在完成时，从句如果用before引导，表示在…之前，句子用过去时
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