A conditional moral rule plus a second premise gives you four possible arguments — two valid, two that only sound valid. Swap the second premise and watch the diagram of possibilities redraw. When the form is broken the machine builds you a counter-model: a world where every premise is true and the conclusion is false.
1 · Pick a conditional rule
2 · Choose the second premise
3 · Now say whether the premises are actually true
What follows
Choose a second premise
no argument yet
P sits entirely inside Q, because the rule says every P-case is a Q-case. Nothing in the rule says the reverse.
Reading the diagramPick a second premise to shade the region you are standing in.