A.T. Robertson Commentary John 15:6

"If a man abide not in me, he is cast forth as a branch, and is withered; and they gather them, and cast them into the fire, and they are burned." — John 15:6 (ASV)

He is cast forth (εβληθη εξω). Timeless or gnomic use of the first aorist passive indicative of βαλλω as the conclusion of a third-class condition (see also verses 4,7 for the same condition, only constative aorist subjunctive μεινητε and μεινη in verse 7). The apostles are thus vividly warned against presumption. Jesus as the vine will fulfil his part of the relation as long as the branches keep in vital union with him.

As a branch (ως το κλημα).

And is withered (εξηρανθη). Another timeless first aorist passive indicative, this time of ξηραινω, same timeless use in Jas 1:11 of grass, old and common verb. They gather (συναγουσιν). Plural though subject not expressed, the servants of the vine-dresser gather up the broken off branches.

Are burned (καιετα). Present passive singular of καιω, to burn, because κληματα (branches) is neuter plural. See this vivid picture also in Mt 13:41f.,49f.