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?