What does natural law teach? There are many ways to summarize the answer. In his detailed discussion of the matter, 1 Girolamo Zanchi states:
The most important things that the law teaches and commands, however, are that all should get what they deserve and serve whom they should serve, both God and human beings. (2)
A little later, he elaborates further:
First, people can protect themselves against any violence or injury. This is a natural reaction for all things. Even trees and plants protect themselves from harm as much as they can. From this instinct comes the idea included in the laws of nations that it is permitted to repel force with force.
Second, human beings can protect not only themselves but also advance their race through the procreation and education of children. This we have also in common with the animals. Because of this impulse, jurists include marriage, reproduction, and rearing children under natural law.
Third, an idea appropriate more for humans than for animals, human beings must recognize their inclination to God and worship him as they do good to those with whom they live, and they must know justice and honesty and turn to them naturally. (10–11)
Of course, from these principles ethical agents can draw more particular practical conclusions given more particular situational contexts.