Why did the Torah write "Zachor" in Yisro and "Shamor" here?
Rashi: Hashem said both words simultaneously as one, and that is how they were heard. 1
Moshav Zekenim: (6:7): Zachor refers to "Hashem created the world in six days [and rested]", and also you should rest on the seventh day!
Emes l'Yakov: Some say that the first Dibros were written on the first Luchos, and the second Dibros were written on the second Luchos. This is unlike Rashi (Sanhedrin 56b), who says that everything in the second Dibros was written on [both sets of] the Luchos! Rather, it was like a Kri and Kesiv. (Every difference between the sets of Dibros, one way was written in the Luchos, and there was a tradition for the other way.)
What is "Ka'asher Tzivcha" referring to?
Seforno: It is referring to keeping Shabbos in the way that the Torah indicated at Marah, where it stated "ve'ha'Yashar be'Einav Ta'aseh, ve'Ha'azanta le'Mitzvosav", 3 an indication that one should spend Shabbos studying Torah and Mitzvos. 4
Rashbam: It is referring to the reason that Hashem stated at Sinai (when He gave Moshe the first Luchos
QUESTIONS ON RASHI
Rashi writes that Hashem first commanded Yisrael about Shabbos in Marah. What is the source? We find that Moshe told them about Shabbos when a double portion of Manna fell, in Midbar Sin (Shemos 16:23)!
Moshav Zekenim (Shemos 20:8): The only place where they received Mitzvos before Matan Torah was in Marah