Main clause: I've known her since ...

In sentences with since (referring to time), we normally use present perfect and past perfect tenses in the main clause.

·         I've known her since 1980. (NOT I know her since ... )

·         We haven't seen Jamie since Christmas.

·         I was sorry when Jacky moved to America; we had been good friends since university days.

However, present and past tenses are also occasionally found, especially in sentences about changes.

·         You're looking much better since your operation.

·         She doesn't come round to see us so much since her marriage.

·         Since last Sunday I can't stop thinking about you.

·         Things weren't going so well since Father's illness.

It's a long time since ...

In British English, present and past tenses are common in the structure

·         It is/was ... since ...

·         It's a long time since the last meeting.

·         It was ages since that wonderful holiday.

American English prefers perfect tenses in this structure.

·         It's been a long time since the last meeting.

·         It had been ages since that wonderful vacation.

3 since-clause: since we were at school

Since can be used as a conjunction of time, introducing its own clause. The tense in the since-clause can be perfect or past, depending on the meaning.

Compare:

-          I've known her since we were at school together.

·         I've known her since I've lived in this street.

-          You've drunk about ten cups of tea since you arrived.

·         You've drunk about ten cups of tea since you've been sitting here.

-          We visit my parents every week since we bought the car.

·         We visit my parents every week since we've had the car.