请教个英语问题，先谢了！

that it had been his own careless actions that had put all their lives at risk. 这是一个宾语从句，that 可以省略。这个宾语从句是一个强调句型，强调句型的特点就是把it is/was/has been/had been 和that 去掉后句子任然成立。去掉就变成his own careless actions had put all their lives at risk.很明显强调的是主语his own careless 。这里的有一个单词你写错了，应该是carelessness,这才是名词

主语.
这个句子可以简单换为:His own careless actions put all their lives at risk.主语为他的鲁莽行动,谓语为:PUT, 宾语:all their lives at risk.