`{True} If x < y then y:= y - x {y>0}` is a Hoare triple, in which `{True}` is the _precondition_ , `{y>0}` is the _postcondition_ , and `If x < y then y:= y - x` is the _command_. The question is whether assuming the precondition holds before the command is carried out, the postcondition is then guaranteed to hold.
Since the precondition is unconditionally satisfied, nothing is assumed about the values of `x` and `y`. In particular, as suggested by the solution key, it may be that `x = y = -1`. If that's the case the _guard_ `x < y` is false, and the assignment `y := y - x` does not take place. Hence, `x` and `y` keep their values, which violate the postcondition `y > 0`.