As someone many years out of practice with Latin I most struggle with assigning objects in the correct cases or with the correct prepositions for my chosen verbs — something most sources aren't all that helpful for.

I understand that prōdesse is mostly used with the dative to specify the beneficiary, but I want to specify the role of the subject, i.e. the manner in which they are of use. Is this just an ablative? Is there a preposition? The only vaguely fitting one I could find was adverbial quā.

So, if for example I wanted to say something like "I am useful to you as a guard" or "they could be useful as sacrifices", how would this look? "Prōsum tibi custōde"? "Prōsum tibi quā custōde"? Completely different? Or is prōdesse unsuited for such a construction?

1 Answer 1


The simplest way is using an apposition:

Prosum tibi custos.
I am useful to you as a guard.

The apposition takes the same case as the implicit subject, ego, the nominative.

This construction can lead to ambiguities, as you cannot distinguish a dog being useful as a guard and a guard being useful as a dog (although word order and context will very strongly hint at the correct interpretation). It can also feel that it lacks emphasis. To get more emphasis and phrasing options, take a look at the various ways of saying "as" emphatically. I'll record some of the ideas here, adapted to your example:

Prosum tibi in loco custodis.
Prosum tibi munere custodis fungens.

Sometimes the specification can be expressed as a prepositional phrase (e.g. "useful in the ceremony/rite/event"). If the specification can be expressed with a verb (like custodire in the case of custos), you can use a participle or a gerund:

Custodiens/Custodiendo tibi prosum.
I am helpful to you by guarding.

If you say prosum tibi custode, my first impression is that you hire a guard and position them to guard your friend.

  • 1
    A terminological note: on the relevant reading (the one you give), custos in Prosum tibi custos is not just an "apposition" but a "praedicativum" (aka "secondary predicate"). The non-predicative appositive reading (what is often referred to as "apposition" tout court) would be something like 'I, the guard, am useful to you'. This said, it should be noted that this grammatical distinction is not always found in Latin grammars.
    – Mitomino
    Apr 6, 2023 at 17:26
  • @Mitomino Fair enough, I was indeed loose in my terminology here. I often try to gauge the level of the asker's Latin proficiency and choose the level of terminological accuracy accordingly. Too much detail can be distracting early on in one's studies. But then again, these posts are read by people with wildly different backgrounds, so it's useful have also the more precise version recorded. // Your comment also highlights an ambiguity in Prosum tibi custos which can be eliminated by a different wording.
    – Joonas Ilmavirta
    Apr 6, 2023 at 18:54
  • 1
    Yes, I agree. My note was just for those readers (perhaps some basic/intermediate learners included) who could be a bit embarrassed when reading that the term "apposition" is equally appropriate for typical cases of this construction (say, Gallos ab Aquitanis Garumna flumen dividit) and your example ([ego] prosum tibi custos).
    – Mitomino
    Apr 6, 2023 at 19:22
  • @Joonas llmavirta: Students are warned not to attempt to use gerunds and present-participles interchangeably, though they both translate to English, "-ing" words. In your last ex., "custodiendo tibi prosum" = "I am useful to you by the guarding.", a verbal noun--gerund-all well and good. Then, "custodiens tibi prosum." = "Guarding I am useful to you.". In English, we wouldn't say it like this; is it good in Latin? Further, shouldn't "custodiens" be agreeing with something? This ex. must be correct but it doesn't look it.
    – tony
    Apr 14, 2023 at 13:30
  • @tony The two constructions are indeed not equivalent but they end up meaning the same in this context. You could say that one is "through the act of guarding" and the other is "by being a guard". It helps a lot if you make a clear distinction in English between the present participle and the gerund; they look alike but mean different things. // Yes, using a participle like this is valid Latin. You can use adjectives like this too. The participle here agrees with the implicit subject ego, just like custos did.
    – Joonas Ilmavirta
    Apr 14, 2023 at 14:17

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