If the question is about time, here is how they are each used: "Since" indicates time from specific point in time, "for" indicates a length of time. So, for example, "I have studied English since 2010" and "I have studied English for 4 years" mean the same as long as you are saying it in 2014. "Since 2010" means from 2010 until now. "For 4 years" indicates a length of time that the action has occured.

Can you be more specific? There are many places/situations where "for" and "since" are used.