how to tell when to use cum temporal and when cum circumstantial

Question

So I have never, ever, ever been able to grasp fully any explanation in any textbook of the difference between cum temporal and cum circumstantial, because the examples they give always seem to contradict the explanations, and I'm wondering whether there's a clearer heuristic than just "the time at which something happens" vs. "the circumstances in which something happens," because a lot of the time I think things should be circumstantial that are actually temporal.

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

• In cum temporal, 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 of cum temporal 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 circumstantial is always susceptible of translation of "since" or "because." But then you get the example Caesar cum loqueretur, ab inimics 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

Proposal: Stop trying to classify all subordinate clauses.

Subordinate clauses with cum can express a number of different things, and they often overlap. Reason, circumstance, and time are very closely related, and drawing boundaries between them is artificial. Do you think a Roman would have classified, consciously or not, your first example in a specific strict category? Classifications are artificial and unnecessary if they have no effect on actual usage.

The only really relevant classification to me is what kinds of cum clauses take indicative and which ones conjunctive. I think looking for a good rule of thumb for moods in cum clauses would make a nice new question, but I will not digress into that here. (The new question was asked later.) If I interpret correctly, your question is about semantic distinctions between causal, temporal, and circumstantial clauses, not between indicative and conjunctive per se. Do correct me if I'm wrong.

Different grammarians will disagree to some extent on the descriptions and classifications of cum clauses.¹ I consider these grammar entries a way of organizing ideas around cum clauses, not as hard facts. I organize my thoughts differently, but I (mostly) get what others are saying. I would argue that an attempt at very narrow and specific classification is ill-advised unless there is overwhelming text corpus evidence to support it.

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¹ For example, Bennett considers circumstantial clauses a subclass of temporal clauses. For another example, the grammar material I was taught with made no mention of circumstantial clauses at all; I think I would classify them as causal or temporal. For more views, ask more people or books.