Galatians 1:8

ἀλλὰ καὶ ἐὰν ἡμεῖς ἢ ἄγγελος ἐξ οὐρανοῦ εὐαγγελίζηται ὑμῖν παρ᾿ ὃ εὐηγγελισάμεθα ὑμῖν, ἀνάθεμα ἔστω.

ἀλλα και - but even (και can mean and or even as in 'even if')
ἐαν - here this is translated should as in 'should it happen', an uncertainty
ἡμεις - we
ἠ - or
ἀγγελος - an angel
ἐξ οὐρανου - out of heaven
εὐαγγελιζηται - preach (proclaim good news), future subjunctive tense/mood
ὑμιν - to you (pl)
παρ' - short for παρα alongside
ὁ - relative, I think the antecedent is the true gospel, which
εὐηγγελισαμεθα  - we preached
ὑμιν - to you (pl)
ἀναθεμα - anathema, banned, rejected state
ἐστω - may it be or may he be, third person imperative.

So, my translation:
But even should we, or an angel from heaven, preach to you alongside the [true gospel] which we preached to you, let it be rejected.