1. Position
[ expression of time + ago]

– I met her six weeks ago (NOT . . . ago six weeks.)
– It all happened a long time ago.
– How long ago did you arrive?

2. Ago is used with a past tense, not the present perfect.

– She phoned a few minutes ago. (NOT She has phoned . . .)
– ‘Where’s Mike?’ ‘He was working outside ten minutes ago.

3. The difference between ago and for

– I went to Spain six weeks ago ( = six weeks before now)
– I went to Germany for six weeks this summer. ( = I spent six weeks in Germany.)

4. The difference between ago and before

– two years ago = two years before now
– two years before = two years before then (before a past time)
– Two years ago, I visited my home town, which I had left two years before.