He is hovering between life and death.

于生死之。

He was balanced on a knife-edge between life and death.

于生死关头。

They wouldn't ask for help unless it were a matter of life and death.

除非是生死攸关事情, 否则们不会请求别人助。

Every time of education on the concept of life and death, should one time of edification, spiritualization and sublimation in mind.

一次生死观教育，应该是一次心灵启迪、心灵净化、心灵升华。