From henceforth (ap' arti). “From now on,” as in Joh 14:7; Mat 23:39; Rev 14:13.
Before it come to pass (pro tou genesthai). Pro with ablative of the articular second aorist middle infinitive ginomai (before the coming to pass).
When it is come to pass (hotan genētai). Indefinite relative clause with hotan and the second aorist middle subjunctive of ginomai, “whenever it does come to pass.”
That ye may believe (hina pisteuēte). Purpose clause with hina and present active subjunctive of pisteuō, “that ye may keep on believing.” Cf. Isa 48:5.
That I am he (hoti egō eimi). As Jesus has repeatedly claimed to be the Messiah (Joh 8:24, Joh 8:58, etc.). Cf. also Joh 14:29 (pisteusēte here); Joh 16:4.