A witness was referred to in 19.35, though he was not explicitly identified. This verse also mentions a witness, who is responsible for writing at least part of the Gospel and is identified as “the disciple whom Jesus loved” (verse 20). At least four problems are involved in the interpretation and translation of this verse:
(1) Who spoke of these things is in the present tense in Greek (Revised Standard Version “who is bearing witness to these things”; Goodspeed “who testifies to these things”), in contrast to the past (aorist) tense of wrote them down. What is the significance of the present tense here? Some commentators believe that the use of the present indicates that the witness on whose testimony the Gospel depends (or the part of the Gospel referred to in these things) was still alive at the time this verse was written; and this interpretation seems to be the one followed by most translations. However, the present tense may simply signal that the disciple’s testimony was considered to be present in the Gospel which he wrote.
(2) What is the precise reference of these things? Does it refer to what immediately precedes (verses 20-23), to all of Chapter 21, to Chapters 1-20, or to the entire Gospel? Since no answer to this question is agreed on by all, the translator should render these things with a term which can refer to any of the possibilities mentioned.
(3) What is the meaning of wrote them down (so also Moffatt, New American Bible; Jerusalem Bible “has written them down”)? Did the disciple write down these things himself or did he cause them to be written down? In 19.19 wrote was used causatively: Pilate did not personally write the inscription on the cross. If wrote is understood causatively in 21.24, does it mean that the disciple dictated the Gospel, that he supervised the writing, or merely that his testimony was the source for the material used (compare 19.35)? If wrote is understood as referring to the testimony behind the Gospel, then it tells us nothing about the authorship of the work. Commentators are sharply divided on the meaning of wrote, and the translator should not favor a particular interpretation in rendering the text.
(4) Who does we refer to (we know that what he said is true)? Some have maintained that the reference is to the disciple who spoke of these things. However, there is no parallel for the third person (he) and the first person (we) being used within the same sentence to designate the same individual. Therefore, it is best to take we as referring to a group which includes the writer of verse 24 but does not include the disciple, whom Jesus loved (verse 20). In languages which distinguish between an inclusive and an exclusive first person plural, the exclusive form should be used here.

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 .