The zero conditional is used to talk about general truths, or things that are almost always true under certain conditions. For example, it’s pretty much always true that when it rains, stuff gets wet.
Zero conditional is easy to form because all the verbs are in present tense. You just use two clauses, one with If/when + simple present verb and the other with another simple present verb, as in, “If students miss an exam, the professor fails them.”
This form can also be used for giving instructions about what to do under certain conditions. For example, “If I’m late for dinner, start eating without me.”
As we saw with Sara’s sentence, the two clauses in the zero conditional can also be reversed. The result can come first and the condition can come second, as in, “My family eats without me if I’m late.” When the clauses are reversed, there is no comma separating them.