Found this paper by:
Charles E. Powell, Ph.D. and John Baima, Clauses in the NT: Interpretation and Translation
https://bible.org/article/%CE%B5%E1%BC% ... ion#P8_317
Powell p.1 wrote:It is often observed in the grammars that the use of εἰ μή with the verb omitted means “except” or “but” and is considered a substitute for ἀλλά.1 This was also common in classical Greek and probably arose as an unconscious abbreviation of the conditional clause because its verb was the same as the main verb in the apodosis.2 The idiom shows three characteristic features. First, there is an ellipsis of the verb in the protasis that is supplied from the principal clause, often the same verb. Second, there is a negative comparison between the two clauses. And third, the protasis always follows the apodosis.3
(Note: the text seemed unformatted so I formatted ajllav with ἀλλά, and eij mhv with εἰ μή)
So it seems like Powell notes as the 3rd characteristics of εἰ μή (if not): as a conditional sentence idiom, there must be an apodosis clause (main clause/consequence) immediately before εἰ μή and a protasis clause (dependent clause/condition) immediately after εἰ μή.
1) So with Mark 6:8, would the following break down be correct?:
( apodosis ) εἰ μὴ ( protasis )
(καὶ παρήγγειλεν αὐτοῖς ἵνα μηδὲν αἴρωσιν εἰς ὁδὸν) εἰ μὴ (ῥάβδον μόνον, μὴ ἄρτον, μὴ πήραν, μὴ εἰς τὴν ζώνην χαλκόν), (SBL)
(And commanded them that they should take nothing for their journey,) save (a staff only; no scrip, no bread, no money in their purse:) (KJV)
2) Now, if I wanted to insert a relative clause into mark 6:8, would the following be a grammatical use of εἰ μὴ?
( apodosis ) (relative clause) εἰ μὴ ( protasis )
(καὶ παρήγγειλεν αὐτοῖς ἵνα μηδὲν αἴρωσιν εἰς ὁδὸν) (ἥν βλέπω) εἰ μὴ (ῥάβδον μόνον, μὴ ἄρτον, μὴ πήραν, μὴ εἰς τὴν ζώνην χαλκόν),
(And commanded them that they should take nothing for their journey), (which I see), save (a staff only; no scrip, no bread, no money in their purse:)