Set the signal generator to 1kHz with and amplitude of approximately 5V. (Make sure the DC offset on your signal generator is zero.) Sketch the input versus the output as a function of time. Note the difference in voltage between the input and output You should see that only current flowing in the positive direction with respect to the voltage of the signal generator. That is, during the positive half of the cycle, the diode is forward biased, thereby letting current through. During the negative half, the diode is reverse biased and no current can pass.