Nooj wrote:καὶ οὐ φοβούμενος μὴ ἁμαρτάνοι τῇ ἑαυτοῦ γνώμῃ πιστεύων, ἐστρατεύσατο ἐπὶ τοὺς Πέρσας, ἵνα τούτους νικήσας τῆς Ἀσίας ἄρχοι.
I don't understand why an optative ἁμαρτάνοι is being used here. The fearing clause needs a secondary tense verb to introduce it in order for an optative to be used, but φοβούμενος is in the present tense...
The main verb of the sentence here is in the past tense: ἐστρατεύσατο
Participles generally express only aspect (eg. completed, simple, repeated), not tense (temporal sequence).
See Smyth §2043:
The tenses of the participle (except the future) not in indirect discourse are timeless, and denote only stage of action. When they stand in indirect discourse and represent the indicative, they denote time relatively to that of the main verb.