talk badly about … behind their back
人のいないところで悪口を言わないで下さい。
Please don't bad-mouth people behind their backs.
相手のいないところで悪口を言うな。
Don't badmouth people behind their backs.
その人のいないところで悪口を言ってはいけない。
You shouldn't speak ill of someone behind their back.
ジムのいないところで悪口を言ってはいけない。
You mustn't speak ill of Jim behind his back.