'since' as time preposition, conjunction and adverb
Michele from France writes:
  Is it correct to use the present perfect after since, for example:

Mr and Mrs Smith have been quarrelling since they've been married. They've been happy since they've lived here.

I was taught that since introduces a date, not a period of time, and no grammar has given me a clear explanation on that question. Thank you.
 
 
Roger Woodham replies:
 
Since is used in a variety of different ways, both with the present perfect and with other tenses.
 
 
'since' as preposition
 
 When it is used as a preposition to introduce a date or a specific time in the past, it is normally used with present perfect and past perfect tenses. It refers to a period of time starting at a particular point in the past and continuing up till now (present perfect) or up until another point in the past (past perfect). Compare the following:
 
I haven't seen my younger brother since 14 July 1998.
They've been on strike since the beginning of April and there's no sign of it ending.
I hadn't visited the area since my childhood days and I noticed last summer how everything had changed.
 
 
'since' as conjunction
 
 Since can also be used as a conjunction, as in your examples, Michele, introducing a clause. The tense in the since-clause can be past or perfect, depending on whether it refers to a point in the past or to a period of time leading up to the present or, in the case of the past perfect, leading up to a point in the past.

Since as a conjunction sometimes combines with ever to make ever since. Note also in these examples that present and past tenses are possible in the main clause as well as the present perfect:
 
We've been patronising this pub (ever) since we've been living in this village.
We've been patronising this pub (ever) since we moved to this village.

Henry's been teetotal since we got married.
Henry's been teetotal since we've been married.

It's only a week since I met him, but we’re very much in love.
It's only a week since we've known each other, but we’re very much in love.

They’re a lot happier since they separated.
They’re a lot happier since they've been living apart.

You’re looking much better since you came out of hospital.
You’re looking much better since you've been out of hospital.

It was in the summer of 2001 that I saw her and it was over 20 years since we had last met.
'Do you realize,' I said, 'it's over 20 years since we last met?'
 
 
'since then' / 'ever since'
 
 Note that since can also be used as an adverb. Since then refers to a particular point in time and ever since to a period of time. Which one we use depends on whether we want to focus attention on the point in time or on the continuing period of time. Compare the following:
 
She left home in 1992 and hasn't contacted us since then. The company started losing money in 2002 and has been in serious decline since then.

The company started losing money in 2002 and has been in serious decline ever since. I took my final exams five years ago and have been working as a doctor ever since.
 
 
    
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