so sometimes the voltage is zero and, I think, that means there is no pushing force for the electrons to flow but current doesn't equal to zero, it will be max(in the mentioned circuit) or any other value(in any other circuit), so what is the pushing force for the electrons at that moment causing them to flow?
The voltage isn't the only "force" pushing the electrons throught the wire. There is also induced vortex-like electric field. Its effect on current is often quantified by a quantity called "electromotive force" or emf. This quantity is work that would be done on elementary positive charge if it was transported along the wire from one to the other terminal. The equation relating current $i$, voltage $U$ and emf is
U + emf = Ri
So if at some instant voltage $U=0$, we have $Ri = emf$; the total driving force is that of induced electric field.
If the resistance of the circuit is low (no resistive load connected), the emf and U have opposite effect on the current most of the time (they are almost in antiphase), so they almost cancel each other and the current is low.