FOR

.example
“I’ve lived here for 8 months”.
“She was waiting for him for five years”.
“We haven’t seen each other for a long time”

SINCE

SINCE
As this period of time, which began at a specific time in the past, continues into the present, we can only use "since" with the perfect tense

AGO

AGO
“It translates as “hace” in Spanish and is used with the simple past tense.