1) "HATFASAH" TO A "DAVAR HA'ASUR"
OPINIONS: The Mishnah discusses cases of Nedarim which a person makes by comparing the object (with Hatfasah) to a Davar ha'Asur. Such Nedarim do not take effect because the verse (Bamidbar 30:3) requires that one compare the object to a Davar ha'Nadur and not to a Davar ha'Asur.
If a person makes a Neder with Hatfasah to a Davar ha'Asur, does the Neder not take effect at all, or does it take effect such that mid'Rabanan he is required to have it annulled?
(a) The RAN proves from the Gemara here that annulment of the Neder is not necessary even mid'Rabanan. The Gemara says that the Mishnah mentions the need for annulment only in the last case of the Mishnah (in which one makes a Neder against one's wife), which implies that in all other cases one does not need annulment even mid'Rabanan. Only when one makes a Neder against his wife does the Neder need annulment, since a husband commonly makes Nedarim against his wife and the Chachamim do not want him to become lax and treat a real Neder (made with Hatfasah to a Davar ha'Nadur) leniently. (The PERISHAH writes that the reason why the Chachamim are stringent in such a case is in order to prevent the husband from refraining from the Mitzvah of Piryah v'Rivyah.) This is also the ruling of the ROSH.
(b) However, the RAMBAM (Hilchos Nedarim 2:13) rules that when a person makes a Neder by comparing the object to a Davar ha'Asur, the Neder needs to be annulled mid'Rabanan if the person is an Am ha'Aretz, regardless of what he prohibited with his Neder. (That is, not only a Neder against one's wife needs to be annulled, but all Nedarim made by being Matfis to a Davar ha'Asur need to be annulled.)
The TOSFOS YOM TOV and VILNA GA'ON (on the Shulchan Aruch) point out that the Rambam's text of the Gemara differs from the text in our editions. According to the Rambam's text, the Gemara (beginning of 14a) does not ask how it could even be suggested that a Neder made with Hatfasah to Avodas Kochavim needs to be annulled mid'Rabanan. Rather, the Gemara asks how it could be suggested that a Neder made with Hatfasah to Avodas Kochavim does not need to be annulled mid'Rabanan, when the end of the Mishnah states that a Neder made with Hatfasah to a Davar ha'Asur needs to be annulled mid'Rabanan. The Gemara, according to the Rambam's text, is saying that the Mishnah itself teaches that all Nedarim made with Hatfasah to a Davar ha'Asur need to be annulled mid'Rabanan.
HALACHAH: The SHULCHAN ARUCH (YD 205:1) rules like the Rambam, that an Am ha'Aretz who makes a Neder with Hatfasah to a Davar ha'Asur needs to annul his Neder mid'Rabanan. The REMA cites the opinion of the RAN and ROSH, that only when one makes a Neder against his wife with Hatfasah to a Davar ha'Asur does he need to annul the Neder, and only when he is an Am ha'Aretz. The Rema adds in the name of the RASHBA that nowadays everyone is considered an Am ha'Aretz with regard to this Halachah.
2) "HATFASAH" TO AN INTANGIBLE ITEM SUCH AS TIME
QUESTION: The RAN writes that when one makes a Neder with Hatfasah by saying, "Meat shall be prohibited to me today like the day on which my father died," in which he is Matfis to the day his father died, he is prohibited from meat only if, on the day his father died, he had made a Neder to prohibit meat. The Ran adds that if, on the day his father died, he had made a Shevu'ah not to eat meat, and now he makes a Neder and is Matfis to that day, the Hatfasah does not work because on the day his father died meat was not prohibited (an Isur Cheftza), but only the person was prohibited from eating meat (an Isur Gavra).
It is clear from the words of the Ran that when a person makes a Neder not to eat meat "like the day on which my father died," the Neder takes effect since there was a Neder on the day his father died. How, though, can a Neder take effect on a day? A Neder can take effect only on a "Davar she'Yesh Bo Mamash," a tangible item. A day is a measure of time and is not a tangible item.
ANSWER: The Gemara (15a) rules that when one makes a Neder to prohibit an intangible item (Davar she'Ein Bo Mamash) and then he violates his Neder, he transgresses an Isur d'Rabanan of Bal Yachel Devaro. The Ran there (end of 15b) understands that this means that his Neder has all of the laws of an ordinary Neder but on a d'Rabanan level.
Here, too, the Ran apparently understands that when the Gemara earlier (12a) and in Shevuos says that the Neder takes effect when one is Matfis to the day on which his father died, he creates a prohibition on a Davar she'Ein Bo Mamash and his Neder takes effect mid'Rabanan. The Gemara here (12a; see Ran on 12a and here on 14a) and in Shevuos (20b) says that even when one made a Neder not to eat meat on the day on which Gedalyah ben Achikam died, he may make a Neder now with Hatfasah to that day -- the Neder takes effect even though meat (and all other food) is already prohibited on that day because of the decree of the Rabanan who declared that day a Ta'anis. Why should the second Isur, the Isur of Neder, take effect if eating on that day is already Asur mid'Rabanan? The reason must be that the Isur Neder mid'Rabanan is an Isur Cheftza, while the Isur Ta'anis of the Rabanan is an Isur Gavra, and an Isur Cheftza can take effect on an Isur Gavra.
(RASHI in Shevuos (20b) cites another Girsa which states that the Neder takes effect on the day on which Gedalyah died because the person is prohibited from eating only mid'Rabanan because of the fast, while the Neder makes him prohibited from eating mid'Oraisa. Rashi rejects that Girsa (even though Tosfos supports it) because even if the Ta'anis would be an Isur d'Oraisa, a Neder would be able to take effect on it. The Ran (18a), who rules like Rashi, also must not have had that Girsa (that the Isur Neder takes effect because it is an Isur d'Oraisa while the Ta'anis is only mid'Rabanan), as he writes that the Isur Neder prohibits meat only mid'Rabanan.)