Since的用法
Since分别有两种涵意：时间和原因。我们可以依照句子语意、结构和时态，决定since在句子中的意思。
1. 时间
在时间的层面上，与「自从」有相似的意思。 Since通常可作为介系词代入一个片语、或连接词代入一个子句、或是以副词单独存在。另外，since后的子句大多是过去式，主要子句为完成式。
连接词
当since用作连接词时，可搭配一段期间或某个时刻，引导时间附属子句，意为「自从…」。附属子句，即 since 子句，的动词通常为过去简单式或现在完成式，而主要子句则使用现在完成式或过去完成式，但以现在完成式居多。如要表示一段期间，since子句该为现在完成式；如要表示某个时刻，since子句便为过去简单式。不论是过去简单式还是现在完成式，意思大致上相同。
【例】I haven’t done any outdoor activities since I graduated from university.
【例】John had been cooking his own dinner since he was twelve years old.
在另一种清况下，当主要子句的结构为”It + be + 时间 + since”，可以以四种不同的句型构成，但意思绝对相同。主要子句的be以现在简单式 is或现在完成式 has been，而附属子句则为过去简单式或现在完成式。在美式英语中，” It has been… since…” 比较常见。
【例】It is ten months since Paul and Linda moved to a new apartment.
【例】It is ten months since Paul and Linda have moved to a new apartment.
【例】It has been ten months since Paul and Linda moved to a new apartment.
【例】It has been ten months since Paul and Linda have moved to a new apartment.
其实在刚刚说明的结构中，be亦可使用过去简单式 was 或过去完成式 had been，但此时 since 子句须使用过去简单式或过去完成式。
【例】It was ten months since Paul and Linda moved to a new apartment.
【例】It had been ten months since Paul and Linda had moved to a new apartment.
当 since 不是引导过去某个动作或事件，而是过去的某个状态并现在一直持续着 ，主要子句及其附属子句都要用现在完成式。
【例】Tom has been sleeping less than 4 hours every day since he has become a doctor.
副词
作为时间副词，since/since then有着「此后，从此，自那以后」的意思。
而ever since能用来加强since和since then的语气。
【例】We graduated from secondary school a year ago and we haven’t reunited since/since then.
【例】They divorced few months ago and haven’t contacted each other ever since.
介系词
作为介系词时，since只能与某一时刻搭配，即是since之后必须接着日期、年份等时间有关的词。时刻能以副词（e.g. yesterday）、名词片语（e.g. the beginning of the match）或动作动词（e.g. I left）表示。意为「自从…，自…以来，从…至今」。
【例】Since yesterday, I haven’t eaten anything.
【例】Since the beginning of the match, she has been cheering loudly for her teammates.
【例】Since I left my previous workplace, it has become much quieter than before.
在某种情况下，我们可以将since从连接词变成介系词—就是当 since 子句和主要子句的主词（subject）是相同时，便可省略 since 子句的主词。这样，本身句中的主动词须改为动名词 ，接在since后。
【例】Since attending tutorial classes, I have become more confident when answering questions during lessons.
（原句：Since I have attended tutorial classes, I have become more confident when answering questions during lessons.）
2. 原因
除了在时间方面，since 亦可作为连接词，引导表示原因的附属子句，意为「既然」。 Since接着的子句通常是现在式，亦能是整个前后子句都为过去简单式。放在主要句子前或后都同样没问题。 Since的运用大多想表达显而易见或已成事实的原因。
【例】Since it is too cold out there, I will just stay home today.
【例】Since the power is out, we should just go to bed directly.
