For and since are two English prepositions that we can use to talk about time in English. They are similar, but we use them a little differently (Be careful! For and since have other meanings as well).
I am living in Medellin since 2011
I study English for 3 hours
I have lived in Medellin since 2011
I have been studying English for 3 hours
The Difference Between For and Since
Here is the important difference between for and since: We use for with an amount of time, but we use since with a point in time.
We use for to express an amount of time (we could also call this a period or time or a duration). Here are some examples of for with amounts of time:
for two years
for 3 hours
for a week
for a long time
for a little while
for as long as I can remember
We use since with a point in time, the time when an action or situation began.
since last week
since December 11
since last fall
since I was born
Ago is a useful word for changing most durations into points in time. That is, two years is a duration and we use it with for. But if we add ago, two years ago is a point in time, and we can say since two years ago. Here are some examples with ago:
since five years ago
since 3 hours ago
since a week ago
Note that using since with ago is a little informal, and better for speaking than formal writing.