| question | answer | correponding form |
| Did Al travel by train? (i.e., Is ψ true?) |
Yes | ψ |
| If he went to Boston | ψ ← φ | |
|
Unless he went to Boston If he didn’t go to Boston |
ψ ← ¬ φ | |
| No | ¬ ψ | |
| Not if he went to Boston | ¬ ψ ← φ | |
|
Only if he went to Boston Not unless he went to Boston Not if he didn’t go to Boston |
¬ ψ ← ¬ φ |