Rabbi Yitzchak addresses a grammatical question in the verse about striking one's parents that has enormous legal consequences. The Torah states: "And if one strikes his father and his mother, he shall be put to death" (Exodus 21:15). The word "and" between "his father" and "his mother" creates an ambiguity. Does the verse mean striking his father or his mother, with each act independently deserving death? Or does it mean striking both his father and his mother, requiring both to be struck before the death penalty applies?
Rabbi Yitzchak argues that "his mother" was added to the verse for purposes of stringency, meaning to extend the death penalty to the case of striking one's mother, not just one's father. If the verse only said "his father," one might conclude that the special severity applies only to paternal assault, since fathers hold a more prominent legal position in many areas of Torah law. Adding "his mother" ensures that maternal assault carries the identical penalty.
But here is the problem. If you read "and" as "and" in its conjunctive sense, requiring both acts to occur, you actually weaken the law instead of strengthening it. Under that reading, a person who strikes only his father is not subject to death. A person who strikes only his mother is not subject to death. Only someone who strikes both parents would be liable. This would make the law more lenient, not more stringent.
Since the Torah added "his mother" to increase the law's severity, not to decrease it, the word "and" must be read as "or." The verse means: one who strikes his father or his mother shall be put to death. Either act, independently, is a capital offense. Rabbi Yitzchak's grammatical analysis ensures that the protection extends equally to both parents.