A. for and since
We use for and since to say how long.?
for three days.
Rachel is in Brazil. She has been there
since Monday.
We use for + a period of time (three days / two years, etc.)
< -------------- for three days ----------------- >
Monday ----- > Tuesday ------ > Wednesday
(past)(now)
for:
three days ten minutes
an hour two hours
a week four weeks
a month six months
five years a long time
B. ago
ago = before now
- Ji Yoo started her new job three weeks ago. ( = three weeks before now)
- ¡°When did Tom go out?¡± ¡°Ten minutes ago¡±( = ten minutes before now)
- I had dinner an hour ago.
- Life was very different a hundred years ago.
We use ago with the past (started / did / had / was, etc.).