A.T. Robertson Commentary John 8:28

"Jesus therefore said, When ye have lifted up the Son of man, then shall ye know that I am [he], and [that] I do nothing of myself, but as the Father taught me, I speak these things." — John 8:28 (ASV)

When ye have lifted up the Son of man (οταν υψωσητε τον υιον του ανθρωπου). Indefinite temporal clause with οταν (οτε + αν) and the first aorist active subjunctive of υψοω, to lift up (Koine verb from υψος, height), used several times in John of the Cross of Christ (3:14; 8:28; 12:32,34). It is unnecessary to render the aorist subjunctive as if a future perfect, simply "whenever ye lift up" (actually lift up, ingressive aorist). In Ac 2:33 the verb is used of the Ascension.

Shall ye know (γνωσεσθε). Future (ingressive aoristic) middle of γινωσκω. Cognoscetis

ex re quod nunc ex verbo non creditis (Bengel). But the knowledge from the facts like the fall of Jerusalem will come too late and will not bring a change of heart. The Holy Spirit will convict them concerning judgment (16:8). For

I am (εγω ειμ) see on verse 24.As the Father taught me (Καθως εδιδασκεν με ο πατηρ). This claim Jesus repeats (see verse 26) and clearly makes on his arrival at the feast (7:16f.). This fact marks Jesus off from the rabbis.