He vowed that he would never do that again.

他发誓绝不再作那样的事。

vow表示发誓。可在vow后加that引导出发誓的内容，也可用vow to do something。
上句可写成：

He vowed never to do that again.
He vowed to do his best to win the game.
他誓言要全力以赴赢得比赛。

编辑:蒋健棠-----