For, Since, Ago
The word “for” is typically used to signify a period of time.
I have lived in Canada for two years.
She has been sleeping for four hours.
The word “since” is typically used in combination with a specific time.
I have been lived in Canada since I was 16 years old.
He has been on a diet since Christmas.
She has been waiting since Tuesday.
The word “ago” is typically used to refer back to a time in the past.
She started cooking three hours ago.
The concert finished 30 minutes ago.