parshanut torah comment - Rashi's proof of the timing of Yisro's conversation with Moshe about judges

Rashi (to Ex. 18:13) cites Mechilta that "the next day," when Yisro saw Moshe judging the people and offered his advice about appointing inferior judges, was the day after Yom Kippur. Part of his proof of this point is:

'שהרי קודם מתן תורה אי אפשר לומר (פסוק טז) והודעתי את חקי וגו

Before the giving of the Torah it was impossible to say (verse 16), “and I make known the statutes, etc.,” [since the statutes had not yet been given].

How so? Previously (commentary to 15:25), Rashi has informed us that Hashem already gave the Jews some mitzvos at Marah: Shabbos, the Red Heifer, and civil laws. (In the commentary to Deut. 5:16, he also adds the mitzvah of honoring parents.) In the weeks before that they had been taught the twenty mitzvos (according to Rambam's enumeration) in Parashas Bo, including the topics of Rosh Chodesh, the Pesach offering, Yom Tov, and sanctifying firstborn sons and animals. Going back even further, they had the seven Noachide laws and all of their corollaries, plus the various mitzvos and customary practices that they had received from the Avos and from Amram (see Rambam, Hil. Melachim 9:1).

So they had quite a respectable amount of Torah law to cover already by the time they arrived at Mount Sinai. Why, then, does Rashi consider it impossible that Yisro could have seen Moshe judging and instructing the people before the Torah was given?

Answer

Nimukei Rashi (who spends several pages discussing this) understands that it was clear to Rashi already from the sequence of events as they are related in Sefer Devarim that this episode of Yisro's advice happened right after Moshe descended from Har Sinai (which is what "ויהי ממחרת" is referring to). However, it is not entirely clear which descending this took place after, for Moshe descended from the mountain several times.

To clarify this, Rashi writes:

הרי קודם מתן תורה אי אפשר לומר (פסוק טז) והודעתי את חקי וגו', ומשנתנה תורה עד יום הכיפורים לא ישב משה לשפוט את העם

In other words, it had to be post-Matan Torah and also with enough time for Moshe to be around judging the people, which is not until he descended the final time on Yom Kippur.

But that which Rashi is learning from "וְהוֹדַעְתִּי אֶת חֻקֵּי הָאֱ־לֹהִים וְאֶת תּוֹרֹתָיו" that this was post-Matan Torah is not because there were no laws to be taught until then, for as you noted, there were laws beforehand as well. Rather, Rashi is learning specifically from the language of "וְהוֹדַעְתִּי", which implies that he was "making known" to the people things that were previously unknown to them.

That is, the laws of Marah and other laws that they already had were already well-known, and even if they needed clarification in minor points or reiteration, this doesn't warrant the language of "וְהוֹדַעְתִּי". It implies that Moshe was delivering laws that were completely knew to the people, which could only be after he had received the Torah.