This verse is best taken as a parenthetical statement by the author of the Gospel. A number of translations, including Good News Translation, place the verse in curved brackets. The verse seems intended to give a background for verse 30.

There may be problems of tense sequence in the parenthetical expression This was before John had been put in prison. An equivalent may be “John was only later put in prison” or “Up to this time John was still not in prison.”

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 .