But when his water ran out, he knew that he would have to do something to save his own life.但是当他的水用完的时候，他知道他必须做一些事情来救自己的命。

辨析：

和这两个短语都有"用完"的意思，但用法不同。run out作不及物动词短语，表示"被用完"， 其主语通常是时间，金钱，食物等无生命的名词。而run out of则是及物动词短语，表示主动意义，主语是人。如：

His money ran out.

He has run out of red ink.