Horav E.M. Shach, Shlita, asserts that there are two distinct demands made by Hashem. First, there is our obligation to personally carry out the mitzvos. Secondly, we are ordained to “guard” the mitzvos by seeing to it that all Jews perform them. It is not sufficient for a Jew to personally observe mitzvos; he is obliged to be mindful that others also perform mitzvos. This, claims Horav Shach, is the meaning of the pasuk “,cav ,t ktrah hbc urnau,” “and Bnei Yisrael should observe/guard the Shabbos.” One must see to it that “Shabbos” is observed by all Jews. It is our imperative to inspire others in the sacredness of Shabbos.
It is noteworthy that the command to observe precedes that of actual performance. Perhaps the Torah is alluding to us that if we are not concerned about overall shmiras Shabbos, then our own personal performance of the mitzvah is lacking. We are thus cautioned to guard the mitzvos by making sure that they are fulfilled by all our fellow Jews.