Since的多种用法
since用作介词、连词和副词，表示时间，当用作连词，表示原因。
Since: 表示时间
我们用since来指代前一个时间点。since作为介词与日期、时间或名词短语连用:
It was the band’s first live performance since May 1990. (since + 日期)
I have been happily married for 26 years, since the age of 21. (since + 名称短语)
since也可用作连词来引入从句:
It’s so long since I saw them. (since + 从句)
Lenny had slept most of the way since leaving Texas. (since + 从句)
He’s been back to the office a few times since he retired. (since + 从句)
Since 与时态
当since在过去的某个时间点引入一个动作或事件时，我们可以在since后使用一般过去时或现在完成时，在主句中使用现在完成时:
They haven’t received any junk mail since they moved house.
They haven’t received any junk mail since they’ve moved house.
我们可以在since之后使用一般过去时、现在完成时或过去完成时来表达it + be + time + since:
It’s been years since I rode a bike. (it’s = it has)
It’s been years since I’ve ridden a bike. (it’s = it has)
It’s years since I rode a bike. (it’s = it is)
It’s years since I’ve ridden a bike. (it’s = it is)
It’d been years since I’d ridden a bike. (it’d = it had)
It 's been years since…在美语中比It 's years since ....更常见。
When since introduces a state in the past that is still continuing in the present, we use a present perfect form of the verb after since and a present perfect form of the verb in the main clause:
当since引入过去的一种状态，而这种状态仍在现在进行时，我们在since之后使用动词的现在完成时形式，在主句中使用动词的现在完成时形式:
Since I’ve been back at work, I’ve been feeling great.
Since + -ing
我们可以用since + -ing形式来表示主语在主句和从句中相同的时间:
Since leaving school, he has had three or four temporary jobs. (Since he left school, he has …)
Since moving from a Chicago suburb to southern California a few months ago, I’ve learned how to play a new game called Lanesmanship. (Since I moved …, I’ve learned …)
Since, since then
当从上下文中理解时间所指时，我们可以使用since或since then作为时间副词:
His father doesn’t talk to him. They had an argument a couple of years ago and they haven’t spoken since. (since they had the argument)
They bought the house in 2006 and they’ve done a lot of work on it since then. (since 2006)
我们用ever since作为since或since then的强化形式:
When I was young, I had a little collie dog, but one day he bit me really badly. I’ve hated dogs ever since.
Since: 原因
我们用since作为从属连词来引入从句。我们用它来给出某事的理由:
Sean had no reason to take a taxi since his flat was near enough to walk to.
Since her husband hated holidays so much, she decided to go on her own.
They couldn’t deliver the parcel since no one was there to answer the door.
Since: 典型错误
我们不用since来表示较长的时间。我们用for:
She was waiting for four hours.
Not: She was waiting since four hours.
我们用since，而不是from连用一个指回到过去某个时间点的从句:
I’ve been swimming since I was three years old.
I’ve been swimming from the age of three.
Not: I’ve been swimming from I was three years old.
我们用since而不是once来引入原因:
I think I should have my money back since I didn’t have what was promised in the brochure.
Not: I think I should have my money back once I didn’t have …
•我们在it ' s a long time之后使用since，而不是ago，当我们指的是某个时间点:
It’s a long time since your last letter.
Not: It’s a long time ago your last letter.