In Cap. X of LLPSI, Ørberg introduces these three sentences:

Cum avis volat, alae moventur.

Cum homo ambulat, pedes moventur.

Cum piscis natat, cauda movetur.

While I understand the general meaning of each, the use of cum and passive verb make it a little more challenging for me.

I think that each more or less means something like:

With wings the bird flies.

With feet the person walks.

With a tail the fish swims.

Is this correct, and is it just the placement of cum at the beginning of each sentence and use of comma that are making it more confusing for me? Shouldn't wings, feet, and tail be ablative?

Is he introducing cum as an adverb, so it would be when a bird flies, the wings move?

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    Related question on cum.
    – Joonas Ilmavirta
    Apr 29, 2021 at 15:53
  • Why would "cum" be an adverb, and not a conjunction? Apr 30, 2021 at 9:06
  • I was basing that off of how cum is listed in various Latin dictionaries, which give it as a preposition and adverb. This is an adverb that is functioning as a conjunction, right?
    – Adam
    Apr 30, 2021 at 15:14
  • @FlatAssembler and Adam: As Cerberus points out in a comment under the answer, cum is a conjunction and a preposition. It does introduce adverbial clauses, but it is a conjunction. Listing it as an adverb strikes me as misleading, as it cannot really be used as one.
    – Joonas Ilmavirta
    Apr 30, 2021 at 15:38
  • Ok, thanks for clarifying that! I was focused on the meaning and wasn't really thinking about what it was grammatically beyond that. It's helpful to know that it's actually a conjunction, though.
    – Adam
    Apr 30, 2021 at 15:48

1 Answer 1


This is indeed the conjunction cum "when" (from Old Latin quom), separate from the preposition cum "with" (from Old Latin com). I'm not quite sure why the verbs are passive EDIT: d_e in the comments has pointed out that moveō is generally transitive, so for a sort of "middle voice" meaning, the passive makes sense:

When a bird flies, its wings move.

As you correctly surmised, the nouns would have to be ablative if it were the preposition "with". But cum is also specifically "with" in the sense of accompaniment ("I ate lunch with a friend"), not "with" in the sense of instrument ("I ate lunch with a fork"). So I wouldn't expect to see it used for feet, wings, etc: birds fly using their wings, not alongside their wings.

  • That makes sense. I'm guessing it's a sort of contrived way to show the passive of moveo.
    – Adam
    Apr 29, 2021 at 15:21
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    If I would been asked to translate it I would rather use the active voice in English. ("The wings are moving"). because the verb moveo is transitive. using the collation se movere instead of the passive, sounds like the wings are an active agent. Kepler for example was using the passive moventur when he was saying the planets are moving.
    – d_e
    Apr 29, 2021 at 15:44
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    @d_e Ah, I didn't know about that use of moveō (as used by Kepler)! A sort of middle-voice-esque usage?
    – Draconis
    Apr 29, 2021 at 15:48
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    @d_e Fair enough! I've edited my answer and might ask a new question about this later today.
    – Draconis
    Apr 29, 2021 at 15:59
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    It has been my experience that the passive and the active reflexive are generally used fairly interchangeably to render a middle sense, at least in classical Latin, apparently without a lot of concern over precisely how active an agent the subject really is in the action. You're just as likely to find se movent and moventur where the sense is clearly, as we would say in English, 'they move.' Perhaps some authors use the two constructions less discriminately but, as I said, this has been my experience. I think a new question about this is a great idea.
    – cnread
    Apr 29, 2021 at 16:15

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