In my book a footnote says coactum is a participle and the phrase is to be translated as “had collected”. Why couldn’t the pluperfect have been used in this instance?
Can you provide more context? The whole sentence and maybe whose work it is.
Remember that the pluperfect of verbs in the passive voice is indeed done through the passive participle (eg, “coactum”), and the verb esse in the imperfect.
Coactum erat — had been gathered.
But the context would help.
Take this with the caveat that my Latin is very bad.
I was reminded of Romance language constructions, and wondered whether this might be late Latin. In which case, I’m wondering whether this might have some helpful explanatory insights: “Habeo + the perfect participle in late Latin,” in J. N. Adams and Nigel Vincent, Early and Late Latin (Cambridge UP, 2016).
Here is the full sentence: Idem facit Caesar, equitatumque omnem ad numerum quattuor milium, quem ex omni provincia et haeduis atque eorum sociis coactum habebat, praemittit qui videant quas in partes hostes iter faciant.
De Bello Gallico - Liber I - 15
coactum is past participle and refers to quem (or rather to equitatum). One could translate quem ex omni provincia et haeduis atque eorum sociis coactum habebat, as
which he had, since it had been collected etc.
In this place the participle is being used as an adjective.
Take the following sentences:
Canem pulchrum habeo. [I have a beautiful dog.]
Pulcher canis lotus est. [The beautiful dog is/was cleaned.]
Pulchrum canem lotum habeo. [I have a clean beautiful dog. / I have a beautiful dog that was cleaned in the past.]
Habeo is being used in its regular function, it just happens to have for an object a word that has a participle being used in its adjectival force. For your context, it’s the “equitatus coactus”:
Equitatem fortem habeo. (I have a brave cavalry unit.)
Fortis equitatus coactus est. (The brave cavalry unit is/was assembled.)
Fortem equitatum coactum habeo. (I have a brave assembled cavalry unit, I have a brave cavalry unit that was assembled in the past.)