since 是英语中使用频率极高的多词性词汇，可作介词、连词和副词，核心含义围绕“时间起点”与“因果逻辑”。其音标为英式 /sɪns/、美式 /sɪns/ 。作为介词时表示“自……以来”，后接时间点；作为连词可引导时间或原因状语从句；作为副词则表示“从那时起”，需与完成时态搭配 。
后接具体时间点（如年份、事件），主句用现在完成时，强调从该时间点持续至今的动作或状态。
例句：She has been studying English since she was 5.（她从5岁起就开始学英语了）
常见错误：不可接时间段（如 since three months 需改为 for three months）
时间状语从句：从句用一般过去时，主句用现在完成时，表“自……以来”。
例：We haven’t seen each other since we graduated.（自从毕业以来，我们就没见过面）
原因状语从句：相当于“既然、因为”，语气弱于 because，强调已知事实。
例：Since everyone is here, let’s start the meeting.（既然大家都到齐了，我们开始开会吧）
用于句尾，指代前文提及的时间点，需与完成时态连用。
例句：He moved to Beijing in 2015 and has lived there since.（他2015年搬到北京，此后一直住在那里）
| 用法场景 | 主句时态 | 从句时态（连词） | 例句 |
|---|---|---|---|
| 从过去持续到现在 | 现在完成时 | 一般过去时 | I have known her since we were children. |
| 从过去持续到过去某点 | 过去完成时 | 一般过去时 | He had worked there since he moved to the city. |
| 副词用法（自那时起） | 现在/过去完成时 | — | She left home in 1990 and hasn’t been heard of since. |
ever since：从那以后（强调持续）
例：I met her in 2018 and have been friends with her ever since.
since then：从那时起
例：We lost touch in 2020, but haven’t spoken since then.
long since：早已，很久以前
例：Bows and arrows have long since been out of use.
since vs. for：
since 接时间点（since 2020），for 接时间段（for three years）
错误：I’ve lived here since three years → 正确：for three years
since vs. from：
表“从现在/将来开始”时用 from，不用 since。
错误：I’ll start working since tomorrow → 正确：from tomorrow
避免 since...to/until：
需用 from...to/until，如 He worked there from 2003 to 2006（不用 since）
自从病后，她很容易疲劳。She's so easily fatigued since her illness.
我们自从毕业就没见过面。We haven’t seen each other since we graduated.
既然你有空，为什么不帮忙呢？Since you are free, why not help me?
他2019年离开后就杳无音信。He left in 2019 and hasn’t been heard of since.
这个部门自2010年成立以来一直很忙。The department has been busy since it was founded in 2010. <cite