Ago vs. Before
We use ago after an expression of time.
- The train went out 10 minutes ago.
Ago indicates a finished time and is normally used with a past tense.
- She telephoned five minutes ago. (NOT She has telephoned five minutes ago.)
The difference between ago and before
Ago is normally used with a simple past to count back from a present moment. Before is used with a past perfect to count back from a past moment.
- I saw him two years ago. (Two years from now.)
- I had seen him two years before I went to England.
Before can be used without a time expression to mean ‘any time before now/then’.
- I think I have seen him before.