My little children, I write these things to you so that you may not sin. If anyone sins, we have a Counselor{Greek Parakleton: Counselor, Helper, Intercessor, Advocate, and Comfortor.} with the Father, Jesus Christ, the righteous.
"May not sin" does not mean "may never ever sin again." Sinning is inevitable for sinners, even forgiven sinners, but in every instance of temptation there is always the possibility that we will not fall (1 Corinthians 10:13). "If" introduces a condition assumed to take place for the sake of the argument (a third class condition in Greek).