He didn't realize that he was wrong until then改为倒装句

not until 位于句首时,要用部分倒装.
具体方法如下：1.先把 not until 从原句中提出来,not until 要写在一起,中间不能有其他词.2.把 Not until 放句首,然后 照搬原句中 Until 后面的内容
3.要知道 not 已经被提出来了,所以剩下的句子就是 肯定句,肯定的陈述句如下：he realized that he was wrong.
现在 只需要 把这个陈述句 改为 一般疑问句 就可以了,
即：did he realize that he was wrong
把上面三点综合起来,得到的句子就是：
Not until then did he realize that he was wrong.
ps:本人是高中一线英语教师,希望我的解答对你有用.