A.T. Robertson Commentary John 6:19

"When therefore they had rowed about five and twenty or thirty furlongs, they behold Jesus walking on the sea, and drawing nigh unto the boat: and they were afraid." — John 6:19 (ASV)

When therefore they had rowed (εληλακοτες ουν). Perfect active participle of ελαυνω, old verb to march (Xenophon), to drive (James 3:4), to row .

Furlongs (σταδιους). Stadia, accusative of extent of space, a little over halfway across, "in the midst of the sea" . It was about forty stadia (six miles) across.

They behold (θεωρουσιν). Graphic dramatic present active indicative of θεωρεω, vividly preserving the emotions of the disciples.

Walking (περιπατουντα). Present active participle in the accusative case agreeing with Ιησουν.

Drawing nigh unto the boat (εγγυς του πλοιου γινομενον). Present middle participle of γινομα describing the process. "Coming near the boat." They behold Jesus slipping closer and closer to them on the water.

They were afraid (εφοβηθησαν). Ingressive aorist passive indicative of φοβεομα, "they became afraid." Sudden change to the regular historical sequence.