consider the cyclic process , like a saquare , the system was returned to its original pressure and volume why the net amount of work done on the system not zero.

let me state a fact always for any process the area coverd within it will give the workdone, since the process is like a square, the area coverd by the square will be the work done hence the net work done is not zero.... also the type of work done in mechanics should not be confused with work done in a thermodynamical process.... try studying the working of a carnot engine to get clarity.....even there the cycle is similar to a square but still net work is there...