"发誓为他报仇"的英文表达是 "to swear revenge for him"。

这个表达是来自英语中的一个常见的成语，即"to swear revenge"。这个成语用来描述一个人决心为自己或他人的损失或受到的伤害而报仇。

下面是一些使用这个成语的英文例句，并附上中文翻译：

"After his brother was killed, he swore revenge on the man who did it." (他的兄弟被杀后，他发誓要报仇。)
"She swore revenge on the company that wrongfully fired her." (她发誓要为被不公正地解雇而报仇。)
"I will swear revenge on anyone who tries to hurt my family." (我会发誓要报仇任何伤害我家人的人。)