I saw reasons why the 39 melachos are listed in the mishnah as the person "The Plower" but why does the Rambam Mishneh Torah perek 7 change the language to the gerund....i.e. plowing for that halacha?
hg, ny usa
A simple explanation is given in Teshuvos Rav Betzalel Ashkenazi (#38) that since the Rambam wants to discuss the different aspects of each Melachah, he mentions the Melachah itself instead of the person who does it.
A more "lomdush" answer is given by the Magen Avos (quoted by the Shevet ha'Levi 7:90). The Mishnah rules like Rebbi Shimon that Melachah she'Ainah Tzerichah l'Gufah is exempt. Accordingly, it is very much dependent on the thought of the person doing the Melachah to determine whether or not he is liable. However, the Rambam (Hilchos Shabbos ch.1) famously rules like Rebbi Yehudah that one is liable for a Melachah she'Ainah Tzerichah l'Gufah. Accordingly, the Rambam wanted to stress that the action is what is important and (usually) not the intent of the person doing it.
All the best,