Understanding the working:

When we give a single-phase supply to the motor, the motor receives electrical power and stator winding is energized creating an emf which in turn produces a stator flux and that emf is induced in rotor creating a rotor flux and now since the single-phase supply has both positive and negative cycles during 0,180 degrees the resultant torque is zero but during positive half +90 degree the forward flux is induced and during negative half -90 degree reverse flux is induced and since the stator and rotor flux is always in phase with each other the forward and reverse flux is in equal and opposite to each other. Thus the net torque is zero and this phenomenon is also known as Double Field Revolving Theory, Hence the motor does not rotate unless and until manually we give a starting torque or else we use any one of the starting methods.