ἐκεῖνο πρῶτον μάνθανε, ὡς......

I am reading a text and meet a question about the structure of a sentence.

Εἰ μὲν ἀθάνατος δοκέεις εἶναι καὶ στρατοῦ ἀθανατου ἄρχειν, οὐδέν ἐστι πρᾶγμα γνώμην ἐμὲ σοὶ ἀποφαίνεσθαι· εἰ δ’ ἔγνωκας, ὅτι ἄνθρωπος καὶ σὺ εἶ καὶ ἑτέρων τοιῶνδε ἄρχεις, ἐκεῖνο πρῶτον μάνθανε, ὡς ἔστι κύκλος τῶν ἀνθρωπείων πραγμάτων, ὅς οὐκ ἐᾷ ἀεὶ τοὺς αὐτοὺς εὐτυχέειν.

I wonder that what “ἐκεῖνο πρῶτον” modifies? Or here it is a noun? I know the general meaning but not sure about the structure.

oh dear. I am amused by myself.

“ἐκεῖνο” is a pronoun in neuter acc. rather than an adjective; “πρῶτον” is adverb here rather than adj. or noun(I subconsciously think that adverb should end in -ως); “-ε” in “μάνθανε” is -ε of singular optative rather than imperfect(past) 3rd singular.

man, what can I say?

μάνθανε is imperative.

‘Oh, Fortuna, blind, heedless goddess, I am strapped to your wheel,’ Ignatius belched…

yes, I mistyped.

You see how chaotic my mind is. lol