> ago = before now (現在之前)

1. Benny started his new job three days ago. (= three days before now.)
2. “When did Betty leave?” “Twenty minutes ago.” (= twenty minutes before now.)
3. I had lunch an hour ago.
4. Life is very different a hundred years ago.

> ago = use in past tense (started/did/had/was …)

* Compare with “ago” and “for”：

1) When did Albert arrive in London?

    He arrived in London three days ago.

2) How long has he been in London?

    He has been in London for three days.