Max Zerwick on John 20:23

  

²³ ἄν = ἐάν. ἀφ-ῆτε aor² subj. -ίημί τινί τι let go; forgive one sth. ἀφ-έωνται: in HGK. ἀφέωμαι supersedes class. ἀφεῖμαι as pf pass.; in pass. the subject is what is forgiven, the person in dat. ἄν τινων (τὰς ἁμαρτίας) if you hold the sins of any. κρατῆτε subj. -τέω conquer; seize; in pres. (subj.) continuous, hold; opp. the specific act of forgiveness (ἀφῆτε), κρατῆτε leaves the situation unchanged §249. κε-κράτηνται pf pass. they remain held, i.e. unforgiven. (Max Zerwick, A Grammatical Analysis of the Greek New Testament [5th ed.; trans. Mary Grosvenor; Rome: Biblical Institute Press, 1996], 345)