(Editor's note: I will be beginning with the 5th mitzvah (commandment), which is where I was up to when I decided to start the project)
The MitzvahThe fifth mitzvah aseh (positive commandment) that the Rambam outlines is our obligation to serve Hashem. Rambam decides to include this mitzvah in his general count, even though he also considers it to be one of the principal commandments, and therefore generally would not be included in the general count. Although Ramban (Nachmonides) will disagree, Rambam says that the reason for its second inclusion is that this mitzvah also carries along with it the obligation of prayer. Additionally, it may come to include the commandment to learn Torah as a form of service to Hashem. The rest of Maimonides' comments on this mitzvah contain the primary (Torah) source for the commandment and supporting texts from the Midrash and Nevi'im (prophets).
The CommentaryRamban, the main commentary on the Sefer HaMitzvot, immediately takes issue with Rambam's insistence that prayer is biblically mandated. The Talmud (Tractate Berachot 21a and Sukkah 38a) makes it quite clear that prayer (meaning Shmoneh Esrei, the eighteen blessing cornerstone of all Jewish prayer) is only rabbincally required to be performed.
To this end, Ramban tries to figure out Rambam from the other extreme. Perhaps Maimonides is trying to say that any type of prayer is a mitzvah, not necessarily the Shmoneh Esrei. But even with this assumption, it is difficult to understand the Rambam since this approach would effectively make all our prayers Torah obligated, and we would therefore run into the same issue we had in the previous paragraph, only in a more extreme way. Even more complicating would be figuring out when this general requirement of prayer would be performed: daily, weekly, once in a lifetime (see Tractate Shabbat 11a)?
Therefore, says the Ramban, it is quite clear there is no obligation of Tefillah, prayer, from the Torah.
On this note, Ramban states that the mitzvah of avodah (service) is that when we do our acts of prayer and service they must be done with all of our heart, i.e. with Kavanah, or concentration. This commandment is telling us to not do a mitzvah without the proper intention and attention, and certainly not to do it if there is a doubt whether you are required to perform that specific mitzvah. This notion is similar to how we are commanded to love Hashem with all of our heart, soul and resources.
The Ramban now moves on to discuss the midrash that Rambam brought in his listing of Mitzvah Five. Ramban asks if this midrash is brought as a support for Rambam's approach to the mitzvah, or is it an actual detailing of what the mitzvah involves? For Ramban, it becomes quite clear that Maimonides brings the midrash to support his outline of the mitzvah, since there is an overwhelming amount of evidence to support both the commandment of prayer in the time of need and Torah learning, as well as the commandment to do Hashem's mitzvot, and therefore a midrash is not needed to detail the individual aspects of the commandment to serve Hashem. With these issues now out of the way, what is the essence of Rambam's mitzvah? Temple service, which is the only place we are able to fully serve Hashem in the proper, Torah, way.