Ago, For and Before
Ago is usually preceded by an expression of time.
The train went out ten minutes ago. (= It is ten minutes since the train went out.)
He died two years ago.
She phoned a few minutes ago.
It happened a long time ago.
Tenses with ago
Ago usually refers to a finished time and hence it is used with a past
tense, not a present perfect tense.
I met her two weeks ago. (NOT I have met her two weeks ago.)
However, a present perfect tense can be used with since…ago.
I haven’t met her since a week ago.
The difference between ago and for
Ago is used to say how long before the present something happened. When used with a past tense for says how long it lasted.
He died three years ago. (= He died three years before now.)
He was ill for several months before he died. (= His illness lasted several
Ago and before
Ago is used to count back from the present. It is used with a past tense and
a time expression. Before is used in the same way to count back from a past moment. It is used with a past perfect tense.
I met that woman at a library three weeks ago. (Not I met that woman at a library three weeks before.)
when we got talking I found out that we had worked in the same office ten years before.
Before can also be used with a present or past perfect tense to mean ‘already’ or ‘before now/then’.
I have seen that film before.