what is the difference between since&for
"since" is used in reference to the beginning of a time period.
"for" is used to describe the length of time.
for example, it's 2:30 in the afternoon and you've been waiting for the bus. the bus was supposed to arrive at 2 o'clock. if you wanted to express the length of time you've been waiting, you'd say "i've been waiting here FOR 3 minutes." if you wanted to tell someone when you started to wait, you'd say "i've been waiting SINCE 2."
so basically "for" refers to length of time and "since" refers to when the activity began.