for, since, ago

She has been there for three days.
                              since Monday.

for + a period of time(three days / two years, etc.).

since + the start of the period(Monday / 9 o'clock, etc.).

ex)
We've been waiting for two hours.
I've lived in Chicago since I was ten years old.

for : ~동안
since : ~부터

ago with the past(started/did/had/was, etc.).

ex)
She arrived in Brazil three days ago.


<in Basic Grammer in use>