A.T. Robertson Commentary John 15:26

"But when the Comforter is come, whom I will send unto you from the Father, [even] the Spirit of truth, which proceedeth from the Father, he shall bear witness of me:" — John 15:26 (ASV)

When the Comforter is come (οταν ελθη ο παρακλητος). Indefinite temporal clause with οταν and the second aorist active subjunctive of ερχομα, "whenever the Comforter comes."

Whom I will send unto you from the Father (ον εγω πεμψω υμιν παρα του πατρος). As in 16:7, but in 14:16,26 the Father sends at the request of or in the name of Jesus. Cf. Luke 24:49; Acts 2:33. This is the Procession of the Holy Spirit from the Father and from the Son.

Which (ο). Grammatical neuter to agree with πνευμα, and should be rendered "who" like ο in 14:26.

Proceedeth from the Father (παρα του πατρος εκπορευετα). "From beside the Father" as in the preceding clause.

He (εκεινος). Emphatic masculine pronoun, not neuter (εκεινο) though following ο.

Shall bear witness of me (μαρτυρησε περ εμου). Future active of μαρτυρεω. This is the mission of the Paraclete (16:14) as it should be ours.