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.

Compare:

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

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.