Thank you very much for explaining it to me.
I'm afraid I missed you on the last part where you explained why the load increases the phase between the two voltages of the primary and secondary coils.
All the alternating magnetic flux in the secondary coil does is to create an induced voltage which its direction is opposite to the magnetic flux's growth direction, and its size is N×dθ/dt, right?
This induced voltage called ε.
If I was correct, then when there's no load, there is still ε exsists, but there is no current since the secondary circuit is an open circuit (no load).
when there's a load, Z, the only difference is that now there is also current I = ε / Z.
I dont see how Z changes the voltage's phase, if the voltage's phase is determined by the primary voltage's phase.