Ago

1) position

expression of time + ago

I met her six weeks ago. (not .. .ago six weeks.) a long time ago

2) tenses

An expression with ago refers to a finished time, and is normally used with a past tense, not a present perfect.

She phoned a few minutes ago. (not She has phoned-...)

'Where's Mike?' 'He was working outside ten minutes ago.' However, a present perfect tense is used with since... ago (as with since + any other time expression).

We've been living here since about eight years ago.

I haven't bought any since a week ago

3) the difference between ago and for

Ago says how long before the present something happened; for (with a past tense) says how long it lasted. Compare:

He died three years ago. (= three years before now)

(not He died for three years, or ... for three years ago.)

He was ill for three years before he died. (= His illness lasted three years.)

4) the difference between ago and before

counting back

Ago is used with a past tense and a time expression to 'count back' from the present; to say how long before the present something happened. Before is used in the same way (with a past perfect tense) to count back from a past moment. Compare:

I met that woman in Scotland three years ago.

(not ... three years before/ before thrce ycars.) When we got talking, I found out that I had been at school with her husband ten years before, (not ... ten years ago.)

'at any time before now/then'

We can also use before, with a present or past perfect tense and no time expression, to mean 'at any time before now/then'.

'Have you been here before?' 'Yes, I was here a year ago.'

As soon as I saw her I knew that I had met her before.