Use for with periods of time: I've been studying English for two years.
for...
three years
two weeks
four days
five hours
ten minutes
three decades
two centuries
Use since with measuring the time after a specific point in time: I've been studying English since 2010.
since.
9:00
1973
Monday
February
last Christmas
I was a child
I graduated from college
As you can see from the last few examples, you don't necessarily need to use a specific date with since. Let's say I graduated from college in 2004 and now it's 2014. I could say:
I've been working at this company since 2004.
I've been working at this company since I graduated from college.
Again, since measures backwards to a specific point in time.
Fact
There was a post office on the Russian space station Mir. Visiting cosmonauts would use unique postal markers to stamp envelopes and other items as having flown aboard the Mir space station .. More >>