[译] 他已掌握了演讲艺术。
[误] He has gained mastery in the art of public speaking.

[正] He has gained mastery of the art of public speaking.
[注] 表示“掌握或精通……”，可以说 gain mastery of... 或 gain mastery over...，但其中的介词 of 或 over 不能换用 in。