When to use cum + subjunctive and when cum + indicative

Question

So I have never, ever, ever been able to grasp fully any explanation in any textbook of when to use "cum" with the subjunctive and when to use it with the indicative, because the examples they give always seem to contradict the explanations. (To me the question "Is it circumstantial or temporal?" only confuses things further, given that it's essentially an order to read the minds of men who have been dead for a couple hundred years.)

The following rules have been suggested to me or occurred to me, but they always seem to overdetermine or underdetermine:

• With cum + indicative, the two clauses have no logical connection with each other. This gets wrong things like Cum Romae estis, agite sicut Romani, which I remember being used as an example in a text. If the people addressed weren't in Rome, the reason to advise them to act like the Romans would no longer exist and therefore there would be no need to say the main clause.
• Cum + subjunctive is always susceptible of translation as "since" or "because." But then you get the example (which I've also seen) Caesar cum loqueretur, ab inimicis interfectus est. He wasn't killed because he was speaking; he was killed because he was destroying the Republic.

Any thoughts on a better heuristic?

Answer 1

Perhaps it's unnecessary to say it but, to be clear, we are dealing not just with the conjunction cum (often quum — the distinction is really immaterial). The word has in fact two kinds of usage; either as a causal particle, or as a normal conjunction. To judge by the numerous articles in many grammars, its use is apparently problematic, as the questioner indicates; but this may well be due to nothing more than over-analysis.

In fact, it's quite possible to single out the cases for which a subjunctive is used, leaving the indicative for everything else. That's because, with a single type of exception, cum In direct speech most often takes an indicative, and always when followed by a present or future tense. Even in the past, the mood is indicative when expressing the time(s) at which an event took place. Cum hostem viderat …, 'whenever he saw an enemy', or cum Ariovistus in Galliam venit, 'when Ariovistus entered Gaul'.

The exception for direct speech is made when simply referring not to time, but to an action in the past that is setting a condition for something that will follow : then, an imperfect or pluperfect subjunctive is used. Caesar is particularly fond of this, in expressions such as eo cum venisset, eo cum ventum esset, and so on.

As a causal particle rather than as a conjunction ('as', 'although', 'since', 'seeing as', etc.), cum takes a subjunctive, even when the reason is made clear. In such cases, it can appear with quippe or utpote (e.g. quippe/utpote quum, 'inasmuch as', and so on). As an example, Quum quisquam operam navare cuperent (Caesar BG 2,25).

Finally: as a conjunction in oratio obliqua, cum invariably takes a subjunctive.

There are, as you might expect, a few rare exceptions to the general rules, which students of syntax delight in identifying : if you come across them, ask yourself if they really matter enough to demand a separate rule. The best antidote to any perceived difficulty in using cum is reading practice, developing a habit of recognition which also serves in translating from English into Latin. And, as an encouragement to practice, think of Horace's exasperation over Homer's mistakes, and remember that not everybody is perfect!