His parents answered is literally “therefore his parents answered and said.” The redundancy “answered and said” reflects Semitic style, while the use of the particle translated “therefore” (oun) is characteristic of John’s style. For stylistic reasons Good News Translation repeats the verb we know (so also Jerusalem Bible). Moffatt restructures by translating “This is our son, and he was born blind; we know that.”

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 .