when necessary

基本解释

I told my students my new telephone number so that all of them called me when necessary.
我把新的电话号码告诉了学生，因此，必要时他们都给我打了电话。
This is not primarily the task of arms, though we will defend ourselves and our friends by force of arms when necessary.
这项任务并非主要由军队来承担，尽管我们会在必要的时候依靠军队来保护我们自己和我们的朋友。
She writes about herself and her pubescent, precocious, and, when necessary, cunning behavior with the same sobriety she uses to describe everything else.
她描写她本人，她的早熟，青春期的困惑，而且在必要的时候，她利用同样节制的狡猾行为去描写所有别的事情。

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

The authorities would act swiftly, and with force when necessary, to quell any such protests.
ECONOMIST: Uzbekistan's president is returned to office, surprise