Elementary
Since, for and ago: talking about periods of time
Jack is in Brazil. He has been there for three days/ since Monday.
> ago = before now (現在之前)
> 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.