When it comes to marriage, American law is straightforwardly binary: You can be married or you can be unmarried. A wedding moves you from one group to the other, and a divorce moves you in the opposite direction, but there is no status in between. As we have seen over the last several months of reading Tractate Yevamot, however, Talmudic law is not so simple. Marriage itself is a two-stage process: A couple is first betrothed, and then, once they have sexual relations or undergo a marriage ceremony, they become fully married. Then there is levirate betrothal: If a man’s brother dies childless, he is bound to his widowed sister-in-law unless and until they either get married or perform the ceremony of chalitza.Actually, I think that American (and British) marriage law is a little more nuanced than that. For example, from what I understand (and I am not a lawyer) most American States, England, and Scotland now regard cohabitation beyond a certain length of time as legally equivalent to marriage in terms of at least most rights and responsibilities. But, yes, Talmudic marriage law is still more nuanced.
Earlier Daf Yomi columns are noted here and links.