The Torah addresses the case of a Hebrew servant whose master gives him a wife during his term of service. In (Exodus 21:4), the verse begins with the word "If" — "If his master gives him a wife." The Mekhilta immediately seizes on this conditional language. "If" means it is optional. The master may give his servant a wife, but he is not required to do so.
But could it actually be mandatory? Could the Torah be commanding the master to provide a wife? Rabbi Yishmael rules this out by connecting the verse to an earlier clause: "If alone he came, alone shall he go out." The word "if" in that clause clearly indicates a possibility, not an obligation. The servant might come alone, or he might not. Since the same conditional structure governs both clauses, the giving of a wife must also be optional.
This distinction matters enormously for the rights of both masters and servants. If providing a wife were mandatory, every master would be obligated to arrange a marriage for his servant — a significant financial and social burden. By establishing that it is optional, Rabbi Yishmael protects the master's autonomy while also clarifying the servant's legal position.
The deeper point is about how the rabbis read the Torah's grammar. A single word — "if" — determines whether a law is a command or a permission. The Mekhilta treats every particle, every conjunction, every conditional phrase as carrying legal weight. Nothing in the Torah is filler. Nothing is casual. The word "if" is not there to make the sentence flow better. It is there to establish a legal principle that governs real human relationships.