'Since' And 'For'

SINCE : Since is used to show exact point of time (Day/Year/Month/Event/Time) in the Past to another moment (in Present or past)
e.g.
1) Rohit has been learning English since 2011.
2) Aniket had been living alone since his father went to another city for job.
   
For : For shows duration. Hours, minutes, days, months or weeks etc.
e.g. : Ajay has been watching T.V. for 5 yrs.
         Ram had been absent for 5 days in first.     
         week of the month.

Since and for can be used as prepositions as well as conjunctions.
'Since' can also be used to mean 'because'
'For' can also be used as 'because'

Since Smita was ill, she did not come to school.
Vivek failed , for he had not studied for a single day.