שָׁמוֹר - Constantly Learning
In his introduction to the volume on Hilchos Shabbos, the Mishnah Berurah cautions that only through constant study of the halachos, can one hope to properly observe them. He quotes the words from the sefer Yaaros Devash, by Reb Yonosan Eibeshutz, who advises that it is impossible not to stumble with chillul Shabbos chas v'shalom, without constantly learning the halachos to achieve clarity.
The Mishnah Berurah also points out that even one who studies a lot of mussar regarding Shabbos observance, if he is not clear in the halachos, it is impossible for him to keep Shabbos properly.
He quotes a pasuk in Mishlei (א ב) לָדַעַת חָכְמָה וּמוּסָר - to know wisdom and discipline. Chachmah means knowledge of Torah. One must study chachmah, which in this case is the halachos of Shabbos, before he learns mussar. Only once one has mastered the halachos of the Torah, can he learn mussar to strengthen his resolve to keep them meticulously. First he has to know the "what" and "how" thoroughly, and only then can he learn the "why" - the Torah concepts regarding the holiness of Shabbos.
