• You use since to say when something started. This is a point in time. e.g. Christmas, 1999, last Monday, 9 o’clock, was ten, etc.
1- Tom’s been waiting for you since three o’clock.
• You use the present perfect in a main clause with since if the action has a result in the present and you’re talking about when it started.
1- We’ve seen Emma twice since the weekend.
• You use the past perfect in a main clause with since if the action is finished.
We’d met several times since that party.
• You use the past simple in a clause after since if the action is finished.
We’d met several times since we were kids.
• We say ‘It’s (a long time/two years etc.) since something happened’ (The question is How long is it since …?)
1- It’s two years since I last saw Joe. (= I haven’t seen Joe for two years/the last time I saw Joe was two years ago)
2- It’s ages since we went to the cinema. (= We haven’t been to the cinema for ages)
3- How long is it since you last saw Joe? (= When did you last see Joe?)
4- How long is it since Mrs Hill died? (= When did Mrs Hill die?)
• We use for when we say a period of time (two hours, six weeks etc.)
1- Marry’s been working here for six months. (not ‘since six months’)
2- I’ve been waiting for two hours.
3- I haven’t seen Tom for three days. (not ‘since three days’)
• You use for to say how long something went on or has been going on.
1- David was at university for four years.
2- Peter’s been studying French for a month.
• You use the present perfect simple or continuous with for if the action has a result in the present and you’re talking about a length of time.
1- Natalia has been at university for six months. (She’s still at university now.)
2- I’ve been reading this report for hours. (I’m still reading it.)
• You use the past simple with for if the action is finished.
Rick was at university for four years. (He’d left university now.)
• It is possible to leave out for (but not usually in negative sentences)
1- They’ve been married (for) ten years. (with or without for)
2- They haven’t had a holiday for ten years. (you must use for)
• We do not use for + all … (all day/all my life etc.)
I’ve lived here all my life. (not ‘for all my life’)
• You use ago after a time period with the past simple.
1- Luke and I met two weeks ago.
2- I lived in Brussels ten years ago.