North & Hillard p.157: Footnote (1): "Moreover in Impossible Conditions, if the verb of the apodosis is possum, debeo, oportet, or a gerundive (or any verb expressing obligation or possibility), it is regularly put in the Indicative. e.g. si patriam perdedisset, interfeciendus erat. If he had betrayed his country, he should have been put to death.
Given the heat generated, recently, over the gerundive, the second clause might be gently massaged into: "it (he)-ought-to-have-been-killed (executed)".
Returning to the point: what exactly does "it is regularly put in the indicative" mean, here--is it mandatory; or, may the student ignore the advice?
This is called into question because when the indicative is deployed, in such circumstances, it is read as a subjunctive; consider:
Ex. 203, Q3: "If the river were not so deep, we might have crossed it on foot." giving: "nisi flumen tam altum esset potuimus id pedibus transire."
Impossible conditions, in the past; and, crossing the river is not a continuous action, in the past; therefore, normally the pluperfect subjunctive is required in both clauses. N & H gave "might" in 2nd clause suggesting imperfect subjunctive, which it cannot be? For me it is "we would have crossed the river"; but the choice of subj. is not the problem. Past-perfect "potuimus" is read as a subjunctive; therefore, what is the purpose of this ruling?