Given circuit is a clipper circuit, which clips some portion of input signal.

Case 1) When Vin < VA  , Dioode gets biased with respect VA ,in this case diode will be reversed biased with respect to VA ,so the doide will act as open switch,Therefore output will be equal to Vin for this specific case.

Case 2) When Vin > VA , Diode gets biased with respect to Vin,in this case diode will be  forward biased with rwspect to Vin ,so it will act as closed switch. And hence output will be equal to Vin-VA.

Again when Vin<VA Case 1 will be followed.</p>

Circuits for both cases:-