How and when to use Since For and Ago in expressions

1. Since

• 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.

2- I’ve been waiting since 10 o’clock. 
3- haven’t seen Johnny since Easter. 
4- Marry’s been working here since April. (= from April until now) 
5- I haven’t seen Rick since Friday. (= from Friday until now)

• 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. 

2- She’s been on a diet since last Wednesday.

• 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?)

2. For

• 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’)

3. Ago

• 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.