Adam Clarke on 1 Corinthians 9:20

Unto the Jews I became as a Jew - In Act 16:3, we find that for the sake of the unconverted Jews he circumcised Timothy. See the note on Act 16:3.

To them that are under the law - To those who considered themselves still under obligation to observe its rites and ceremonies, though they had in the main embraced the Gospel, he became as if under the same obligations; and therefore purified himself in the temple, as we find related, Act 21:26 (note).

After the first clause, to them that are under the law as under the law, the following words, μη ων αυτος ὑπο νομον, not being myself under the law, are added by ABCDEFG, several others; the later Syriac, Sahidic, Armenian, Vulgate, and all the Itala; Cyril, Chrysostom, Damascenus, and others; and on this evidence Griesbach has received them into the text.