A.T. Robertson Commentary 1 Corinthians 9:20

"And to the Jews I became as a Jew, that I might gain Jews; to them that are under the law, as under the law, not being myself under the law, that I might gain them that are under the law;" — 1 Corinthians 9:20 (ASV)

As a Jew (ως Ιουδαιος). He was a Jew and was not ashamed of it (Acts 18:18; Acts 21:26).

Not being myself under the law (μη ων αυτος υπο νομον). He was emancipated from the law as a means of salvation, yet he knew how to speak to them because of his former beliefs and life with them (Galatians 4:21). He knew how to put the gospel to them without compromise and without offence.