For the Father as one who testifies on behalf of the Son, see 5.37.

Since the Father who sent me must be rendered in many languages as “my Father, who sent me,” note that the clause who sent me becomes non-restrictive, that is, it simply describes an aspect of the Father, without indicating which particular Father is to be understood. In many languages such a clause must be treated quite differently from a normal relative clause which specifies a particular individual, for example, it may be necessary to say “my Father (he is the one who sent me).”

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 .