"I will tell him tomorrow when he is calmer."
Translation:Je le lui dirai demain quand il sera plus calme.
The point is to learn that the French requires this even when it's implicit in the English. In English, we would often omit the "it" although we can put it in if we feel like it. It seems that in French you can't have an indirect object without a direct object so you need the le. Happy to be corrected by a native French speaker but that's what I've gleaned so far from these lessons.