A.T. Robertson Commentary John 8:51

"Verily, verily, I say unto you, If a man keep my word, he shall never see death." — John 8:51 (ASV)

If a man keep my word (εαν τις τον εμον λογον τηρηση). Condition of third class with εαν and constative aorist active subjunctive of τηρεω. Repeated in verse 52. See verse 43 about hearing the word of Christ. Common phrase in John (8:51,52,55; 14:23,24; 15:20; 17:6; 1 John 2:5). Probably the same idea as keeping the commands of Christ (14:21).

He shall never see death (θανατον ου μη θεωρηση εις τον αιονα). Spiritual death, of course. Strong double negative ου μη with first aorist active subjunctive of θεωρεω. The phrase "see death" is a Hebraism (Psalms 89:48) and occurs with ιδειν (see) in Lu 2:26; Hebrews 11:5. No essential difference meant between οραω and θεωρεω. See Joh 14:23 for the blessed fellowship the Father and the Son have with the one who keeps Christ's word.