The Torah says, "When you lend money to My people" (Exodus 22:24), using the Hebrew word "im," which normally means "if." This would seem to make lending optional, a generous act you might choose to perform but are not required to. The Mekhilta rejects that reading entirely. Here, "im" does not mean "if." It means "when." Lending to the poor is mandatory.
The proof comes from Deuteronomy: "Lend shall you lend to him" (Deuteronomy 15:8), where the doubled verb, a characteristic Hebrew construction for emphasis, makes the obligation unmistakable. The Torah is not suggesting that you might consider helping someone in need. It is commanding you to open your hand and extend the loan.
This distinction between optional and mandatory carries enormous weight in Jewish law. If lending were merely permitted, a person could decline without sin. But since it is commanded, refusing to lend to someone in need is a violation of the Torah, as real and serious as violating any other commandment. The Mekhilta understood that generosity left to human discretion would fail. People find reasons not to lend. They worry about repayment, they judge the borrower's worthiness, they protect their own comfort. The Torah cuts through all of those excuses with a single grammatical ruling: "im" here means "when," not "if." There is no opt-out clause for compassion. When you encounter poverty, you lend. Period.