substantive usage of the participle in Herodotos

τὰ μέν νυν ἓξ ἔτεα τῶν ἕνδεκα Σαδυάττης ? Ἄ?δυος ἔτι Λυδῶν ἦ?χε, ? καὶ ?σβάλλων τηνικαῦτα ?ς τὴν Μιλησίην τὴν στ?ατιήν: Σαδυάττης οὗτος γὰ? καὶ ? τὸν πόλεμον ἦν συνάψας:
1.18.2

Can a participle used substantively be separated from its article by ἦν? Or is the participle συνάψας not being used substantively here?

No, it isn’t substantive. It is modifying Saduattes. “This Saduattes was the one who took up…”

But translating it as “the one who took up” is still translating the participle as a substantive in predicate relation to Saduattes.

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Thanks for the reference. I haven’t found any references in Smyth to this issue, and, as you stated, I think it is relatively rare.

But it isn’t a substantive if it is modifying another noun. Then it is a predicate adjective.

It’s not modifying another noun. It’s functioning as a predicate noun, not as a predicate adjective, i.e. “…for this Saduattes is the one also having commenced the war.”