Imposing Conditions On The Conditional

Question

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?

Answer 1

There are a couple issues to sort out.

First, the past tense of certain verbs such as debeo, possum, oportet, etc., can carry counterfactual force, even when in the indicative. This can occur in independent clauses with no expressed protasis.

Bonus vátes poteras esse, nam quae sunt futura dicis. (Plautus, Miles 911)
You would have made a good prophet (but you aren't one), since you say what will happen.

So, conditional sentences are merely one instance where this linguistic feature applies. There is a full treatment of the indicative in counterfactual conditions in A&G 517. But to give a short answer, the indicative is not mandatory. Both moods are found.

Now, for the specific example sentence you raise (203.3), the English wording is rather confusing. I believe the sentence is trying to communicate continuous state. That is, the river both was and is too deep, with the result that we could not and still cannot cross. If this understanding of the English is correct, the imperfect is called for.

Nisi flumen tam altum esset, pedibus transire poteramus.

[Note: Many languages evidence interchange between the past tense and subjunctive of certain verbs that communicate potentiality or obligation. Greek is similar to Latin; ἐχρῆν, ἔδει, ὤφελον, etc. Modern languages often do this with modal auxiliaries. In English, "could" can be past tense (In high school, I could dunk a basketball) or subjunctive (I could dunk a basketball right now if I wanted to).]