This is the question about the SCR/thyristor, right? So basically, a couple of fundamentals about the SCR/thyristor:
-They act similarly to a diode (current flows only in one direction), BUT they do not conduct until there is a voltage or current pulsed on the SCR's or thyristor's gate terminal.
-Once the SCR or thyristor conducts, it will continue to conduct current until the voltage/current between the cathode and anode reaches zero.
-After the SCR or thyristor stops conducting, it will need a current on the gate signal to start conducting again.
So in this problem, the gate current signal first appears at alpha. The thyristor therefore first conducts anode current at alpha. Anode current is zero before alpha, but then it follows a sine wave curve until the anode current hits zero again at pi. The thyristor only conducts anode current when it is forward-biased, which is from zero to pi. It does not conduct any anode current when it is reverse-biased, which is from pi to 2 x pi.