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?