Rashi explains that in regard to Succah which is di'oraisa the word "pasul" is used because this was derived from the Torah at Sinai before the Mishnah was taught and it wasnt dont accoring to TOrah and Halacha, while for the mavouy, a Takanah was taught because this was its first teaching and it was not shayach to use the lashon of Pasul.
My question is the following: Dont we learn that TOrah She'Beal Peh was given prior to Torah She'Bechtav at Har Sinai and the Mishnah was thus taught first? If the Mishnah was taught first, then how does this fit in with Rashi's explanation-it seems that it contradicts it!!!!?????
There are no sources that say that Torah sh'Ba'al Peh was given prior to Torah sh'Bichtav. Rather, as Hash-m taught the Torah sh'Bichtav to Moshe Rabeinu, He also taught him what it means (i.e., Torah sh'Ba'al Peh). (Perhaps you are thinking of the Magen Avraham and Beis ha'Levi regarding Shavuos, who explain that the "Torah she'Ba'al Peh" was given the first day of Shavous, and the Torah she'Bichsav" was given the second day. However, they only mean that the right to determine how to understand the Torah was given on that day, not the wording of Torah she'Ba'al Peh.)
Furthermore, the text of the Mishnayos as we have them was not compiled until Rebbi Yehudah ha'Nasi (Rabeinu ha'Kodesh). Although the facts and details were given over from Hash-m to Moshe and transmitted from rebbi to Talmid for many generations before Rebbi Yehudah ha'Nasi, it was his decision how to redact them in written form. He chose not to use the word "Pasul," but rather to express the Takanah, when writing the Mishnah in Eruvin, for the reason the Gemara here gives.
M. Kornfeld