Who determines what work I can do? I have a broken left arm & i am a mechanic. I'm supposed to go back to work even though they said they don't have light duty.

did your treating dr release you to work? you have to do what your dr tells you, if you get released light duty and your employer can't accomidate you, you should be able to collect ttd benefits through wc.
you do.
the doctors and employers and carriers can only give you options and advice.
of course, the comp laws of your state determine if disability will be paid while you are off work.
generally no disability payments can be made for periods not authorized by a doctor.
where modified work is concerned usually the doctor and employer generally use trial and error to find the right set of modifications.