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the Comforter
This refers to the Holy Spirit. See how you translated this in John 14:16.
will send ... from the Father ... the Spirit of truth ... he will testify about me
God the Father sent God the Spirit to show the world that Jesus is God the Son.
Father
This is an important title for God. (See: rc://en/ta/man/translate/guidelines-sonofgodprinciples)
the Spirit of truth
This is a title for the Holy Spirit. AT: "the Spirit who tells the truth about God and me." (See: rc://en/ta/man/translate/figs-explicit)
You are also testifying
Here "testifying" means to tell others about Jesus. AT: "You also must tell everyone what you know about me." (See: rc://en/ta/man/translate/figs-explicit)
the beginning
Here the "beginning" is a metonym that means the first days of Jesus' ministry. AT: "from the very first days when I began teaching the people and doing miracles" (See: rc://en/ta/man/translate/figs-metonymy)