Isaiah said this is literally “These things Isaiah said.” But once again “these things” is best translated this for English speakers, as in most translations.

Because (hoti) appears in some Greek manuscripts as “when” (hote). The manuscript evidence is heavily in support of the reading because, and in the context it seems more probable that a scribe would have changed because to read “when” than vice versa. Most modern translations (with the exception of Jerusalem Bible and Luther) follow the reading because. Note the similar change in verse 17.

Jesus’ glory is literally “his glory,” but the pronominal reference is ambiguous for the English reader, and for this reason Good News Translation makes it explicit. Note the ambiguity of New English Bible: “Isaiah said this because he saw his glory and spoke about him.” According to the English sentence structure, it would be natural to refer the pronouns to God, who is mentioned in the last part of the preceding verse. However, John is affirming that Isaiah had seen Jesus’ glory, and this information should be made explicit (note Die Bibel im heutigen Deutsch “Isaiah spoke here of Jesus. He said this because he had seen his glory”). New American Bible is similar to Good News Translation, and Jerusalem Bible makes “Jesus” explicit by reading “Isaiah said this when he saw his glory, and his words referred to Jesus.”

Certain problems are involved in the translation of verse 41 because the temporal relations are not made explicit in the Greek text. Because he saw Jesus’ glory could mean that Isaiah saw in his own day the pre-incarnate glory which Jesus had. However, it is better to understand this clause as referring to Isaiah’s prophetic vision of the glory that Jesus would have as the result of his death and resurrection. One may translate, therefore, “he saw ahead of time the glory that Jesus would have later” or “… how wonderful Jesus would be.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .