‘Ach beasor lachodesh’ - On the tenth day…..

Ramban –explains that the word ach means surely. On Yom Kippur Hashem will surely act with mercy and will mochel, forgive, the misdeeds of Klal Yisrael.

Rabeinu Bachayee---explains ach to mean but/yet. Hashem gave us Rosh Hashana to bring judgement on people for their wrong doings, yet when it comes to Yom Kippur he seals the judgement with mechila (forgiveness).

The parsha speaks about Shabbos and Yom Tov. The Ramban teaches us from the pisukim in the parsha that when Shabbos comes out on Yom Tov you still cannot do any melacha. The reason you are allowed to do melacha on Yom Tov is not because it is less holy then Shabbos, rather the reason is out of repect for Simcha on Yom Tov. The Torah therefore lets you do melacha to accomplish this simcha on Yom Tov. Logically, it would seem that when Yom Tov coincides with Shabbos maybe you could still do melacha to help enhance the simcha. Hence the posuk comes to tell us this is not true because Shabbos is greater.