This sentence comes from Caesar's De Bello Gallico (emphasis mine in the part I'm trying to understand):

Dum haec a Caesare geruntur, Treveri magnis coactis peditatus equitatusque copiis Labienum cum una legione, quae in eorum finibus hiemaverat, adoriri parabant, iamque ab eo non longius bidui via aberant, cum duas venisse legiones missu Caesaris cognoscunt.

In the expression cum duas venisse legiones, I find an accusative duas legiones and a perfect infinitive, venisse, so I suppose it's an accusativus cum infinitivo construction. My problem is that I've never encountered such structure used with cum. What's the role of cum in this case?

1 Answer 1


It's a temporal cum clause. See section 545 in Allen and Greenough.

longius bidui...aberant cum ... cognoscunt. ("they where not farther away than two days journey... when they noticed that ...)

Cum can indicate something alongside something else either in space or in time. Here, cum conjunctively indicates that the action of the final clause happened at the same time that they were two days away from wherever.

  • I see... So, in some sense, cum introduces the verb cognoscunt and has nothing to do with the accusativus cum infinitivo.
    – Charo
    Nov 15, 2023 at 15:04
  • @Charo Right, it is simply a conjunction like the English "when" is used conjunctively. Nov 15, 2023 at 15:48

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