since 是一个使用频率非常高的单词，它的词性丰富，可以作介词、连词和副词，而且用法也非常丰富，特别是完成时的连用，但是现实中往往有很多人只是用了冰山一角，而且依葫芦画瓢的很多。
since 用来表示某段时间的起点，还可用来指某事发生的最后时间，或某段时间内某事没有发生，例如：
She has been with the group since it began.
该集团创建之初她就在那儿工作。
She hadn't eaten since breakfast.
早饭后她一直没吃过东西。
一、since 表示从过去的某个时间点开始直到现在，主句用现在完成时，since 作介词时后面接时间点（一个具体的年月日或特殊的日子），作连词时接从句，例如：
1、since 后面跟时间点或从句，若后面接从句，则一般用一般过去时，例如：
a、自从你上次打电话以来，他一直不在办公室。
正确：He has been out of the office since you last phoned.
错误：He is out of the office since you last phoned.
b、正如你所回忆起来的，巴黎办事处从这个项目开始就一直在协助我们。
正确：As you may recall, the Paris office has assisted us with this project since it started.
错误：As you may recall, the Paris office assists us with this project since it started.
c、琼斯先生自2016年9月起受雇于本公司。
正确：Mr Jones has been employed by the Company since September 2016.
错误：Mr Jones is employed by the Company since September 2016.
2、当 since 从句描述的是一个从过去持续到现在的一个状态时，since 之后的从句用现在完成时或现在完成进行时，例如：
The department has been extremely busy since we have had an office in Mumbai.
自从我们在孟买设立办事处以来，这个部门一直非常忙。
解析：since 后面的现在完成时表示是从过去的某个时间一直到现在办事处已经完成设立，强调动作的持续性。若后面用一般过去时表示的则是在过去的某个时间办事处已经完成设立，强调动作的完成。
I’ve been feeling much better since I’ve been taking more exercise.
自从我多锻炼以后，我感觉好多了。
解析：since 后面的现在完成时表示是从过去的某个时间一直到现在我一直在健身，强调动作的持续性。若后面用一般过去时表示的则是在过去的某个时间我健身，强调动作的完成。
3、since 还有一种结构：since V-ing，其中 since 这里是介词，例如：
自1998年以初级助理身份加入律师事务所以来，他已成为该领域最受尊敬的律师之一。
Since he joined the firm as a junior associate in 1998, he has become one of the most highly regarded lawyers in his field.
Since joining the firm as a junior associate in 1998, he has become one of the most highly regarded lawyers in his field.
二、since 表示从过去的某个时间点开始直到过去的另外一个时间点，主句用过去完成时，since 作介词时后面接时间点（一个具体的年月日或特殊的日子），作连词时接从句，例如：
1、since 后面跟时间点或从句，若后面接从句，则一般用一般过去时，例如：
In 2013 I was asked to direct the company’s promotional film. I had not done anything like that since I worked in TV advertising in 1993.
2013年，我被要求执导公司的宣传片。自从1993年我在电视广告公司工作以来，我从未做过这样的事。
When I arrived I realised that they had been in the meeting since 9 am.
当我到达时，我意识到他们从早上9点就开始开会了。
2、当 since 从句描述的是一个从更早的过去持续过去的某个时间点的一个状态时，since 之后的从句用过去完成时，例如：
It had been 20 years since I had directed any kind of film.
我已经20年没有执导任何一部电影了。
三、可以在 since 前面加 ever 来强调 since，例如：
Our two senior partners have known each other ever since they were at primary school.
我们的两个老搭档从小学就认识了。
He moved to London in 1994 and he has been living there ever since.
他1994年搬到伦敦，从此一直住在那里。
四、since 不用来表示现在、将来或一般的时间描述，用 from，例如：
现在：
我们希望办事处从现在开始运作，最迟在六月底开始运作。
错误：We want the office to start operating since now, or by the end of June at the latest.
正确：We want the office to start operating from now, or by the end of June at the latest.
将来：
今天晚上7点起我就在酒吧里。
错误：I’ll be in the bar since 7 o’clock this evening.
正确：I’ll be in the bar from 7 o’clock this evening.
一般的时间描述：
许可证发放程序自提交申请之日起不得超过90天。
错误：The licence-granting procedure should not take longer than 90 days since the submission of the application.
正确：The licence-granting procedure should not take longer than 90 days from the submission of the application.
五、since 不用于一个时间段，用 for，例如：
如果承租人拖欠三个月的房款，房东可以立即终止本协议。
错误：The Landlord may terminate the Agreement with immediate effect if the Tenant is in arrears with payments since three months.
正确：The Landlord may terminate the Agreement with immediate effect if the Tenant is in arrears with payments for three months.
虽然我们通常希望在提出申请后6周内作出决定，但索赔人现在已经等待了10周。
错误：Although we would usually expect a decision within six weeks of making an application, the Claimant has now been waiting since ten weeks.
正确：Although we would usually expect a decision within six weeks of making an application, the Claimant has now been waiting for ten weeks.
六、不用 since … to/until，应该用 from … to/until，例如：
他从2003年到2006年在微软工作。
错误：He worked at Microsoft since 2003 to 2006.
正确：He worked at Microsoft from 2003 to 2006.
他从胜任的那一刻起一直在这家公司工作到退休。
错误：He worked at the company since the moment he qualified until his retirement.
正确：He worked at the company from the moment he qualified until his retirement.
七、since 作副词，译为“自…以后，从…以来；此后，后来”，与现在完成时或过去完成时连用，例如：
He left home two weeks ago and we haven't heard from him since.
他两周前离家外出，我们至今还没有他的音信。
We were divorced two years ago and she has since remarried.
我们两年前离了婚，之后她又再婚了。
八、since 可以表原因，跟 because 类似，译为“因为，由于，既然”，例如：
We thought that, since we were in the area, we'd stop by and see them.
我们想，既然到了这个地方，就该顺便去看看他们。
I'm forever on a diet, since I put on weight easily.
我永远都在减肥，因为我很容易长胖。
但是为了避免与 since 常用的引导时间状语从句的用法冲突，引导原因状语从句的时候最好用 because 代替，例如：
We have analysed the tax aspects of the transaction since we were instructed to do so.
解析：此句的 since 就很容易被人误解，可能有以下两种翻译：
翻译1：因为我们被指示这样做，所以我们已经分析了交易的税务方面。
翻译2：自从被指示要这样做以来，我们已经分析了交易的税务方面。
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