The government, when necessary, has recourse to the armed forces.

政府在必要时可以动用军队。

《牛津词典》

They are not afraid to tackle the issues or let the sparks fly when necessary.

他们不怕应付这些问题, 必要时也不惜进行唇枪舌剑的激烈争论。

《柯林斯英汉双解大词典》

Only flush the toilet when necessary: this means to not use the toilet as a garbage to avoid wasting water.

只在必要时冲厕所：这意味着不要把厕所当作垃圾桶使用以避免浪费水。

youdao

The law established the idea that the government could control industries, when necessary,for the public good.

VOA: special.2010.05.06

Have we at least found something interesting,necessary, unique to death when we say,Nobody can die my death for me.

我们是否至少找到一些有趣，必要，独特的东西，当我们说“没有人能代替我死亡。

耶鲁公开课 - 死亡课程节选

But sometimes it's necessary when you have really big course load

但是有时候课程量很大的时候这是必要的，

开放式的课程？ - SpeakingMax英语口语达人

Lucas will act as the creative consultant, to fill in the blanks when necessary.

FORBES: Move up http://i.forbesimg.com t Move down

The Fed has consistently demonstrated its unwillingness to take the appropriate actions when necessary.

FORBES: Bernanke: 60 Minutes, 2 Big Lies

When necessary, this passageway is kept open using medication in babies with tricuspid atresia.

CNN: Tricuspid atresia