She has been with the group since it began.
She hadn't eaten since breakfast.
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：
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.
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.
正确：Mr Jones has been employed by the Company since September 2021.
错误：Mr Jones is employed by the Company since September 2021.
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.
I’ve been feeling much better since I’ve been taking more exercise.
3、since 还有一种结构：since + V-ing，其中 since 这里是介词，例如：
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：
In 2021 I was asked to direct the company’s promotional film. I had not done anythinglike that since I worked in TV advertising in 1993.
When I arrived I realised that they had been in the meeting since 9 am.
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.
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.
He moved to London in 1994 and he has been living there ever since.
四、since 不用来表示现在、将来或一般的时间描述，用 from，例如：
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.
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.
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：
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.
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：
错误：He worked at Microsoft since 2003 to 2021.
正确：He worked at Microsoft from 2003 to 2021.
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.
He left home two weeks ago and we haven't heard from him since.
We were divorced two years ago and she has since remarried.
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.
I'm forever on a diet, since I put on weight easily.
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：