Rabbi Yishmael taught that the word "if" in the Torah generally means something is optional — except in three specific cases where "if" actually means "when," making the instruction mandatory.
The first exception appears in (Exodus 20:22): "And if an altar of stones you make for Me." This sounds optional — "if" you happen to build a stone altar. But Rabbi Yishmael argued it was mandatory. How do we know? Because (Leviticus 2:14) provides a parallel: "And if you offer an offering of first-fruits." This also sounds optional, but a companion verse — "You shall offer the offering of your first-fruits" — uses mandatory language, confirming that the "if" means "when."
This interpretive principle had enormous practical consequences. The difference between "if" (optional) and "when" (mandatory) could determine whether an entire category of offerings was required or merely suggested. Rabbi Yishmael's three exceptions created binding obligations out of verses that appeared, on their surface, to grant permission rather than impose duty.
The teaching also reveals how carefully the rabbis tracked every particle of Hebrew grammar. A two-letter word — "im," meaning "if" — could carry the weight of an entire legal category. Most of the time it meant one thing. In three cases it meant the opposite. Identifying those three exceptions required cross-referencing multiple passages and constructing chains of inference. Nothing in the Torah's language was taken at face value without checking it against the broader textual system.