John 3:12

εἰ τὰ ἐπίγεια εἶπον ὑμῖν καὶ οὐ πιστεύετε, πῶς ἐὰν εἴπω ὑμῖν τὰ ἐπουράνια πιστεύσετε;

If I said to you the earthly things and you do not believe, how will you believe if I were to tell you the heavenly things


 * τὰ ἐπίγεια and τὰ ἐπουράνια are adjectival substantives. The earthlies and the heavenlies. In translation we add "things".
 * Note the prepositional prefixes ἐπί for γεια (out of earth) and ἐπ for ουράνια (out of the heaven)
 * ἐὰν - if... expects a subjunctive
 * εἴπω - is the subjunctive. I were to say.
 * πιστεύσετε - regular ol' future active indicative 2P pl. Will believe.