Nedarim, Chapter 2, Mishnah 2. This mishnah lists examples of cases in which the declaration does not render the item forbidden, whether because of the type of neder the person used or because of the type of shevuah.
Neder formulations that do not take effect:
"Korban lo ochal lach" - whatever I do not eat of yours should be like a korban. This is not a neder, since the prohibition is placed upon what he is not going to eat at all, while the food he actually does eat from him remains permitted. For the neder to take effect, he would have to forbid the thing he regularly eats; there is no meaning in forbidding something he is not eating anyway.
"Korban she'ochal lach" - at first glance this sounds like a valid neder formulation: let that which I eat of yours be like a korban. But the Gemara explains that what he actually said was "haKorban she'ochal lach," and this phrasing sounds like an oath sworn by "chei korban," by the life of the korban. Swearing by the life of something is a form of oath, just as Yosef in Egypt used the expression "chei Pharaoh," by the life of Pharaoh, every time he wished to swear to his brothers about something that was not true. It emerges that this is not a neder at all but an oath sworn by a korban, and therefore it does not take effect.
"Lo korban she'ochal lach" - let that which I eat of yours not be a korban. There is a possible inference contained within the statement itself: from the negative one hears that the food he was accustomed to eat would indeed be a korban, and this is the principle "mikelal lav atah shomea hen" - from a statement made in the negative one can understand the positive, and vice versa. Nevertheless the mishnah rules that it is permitted, because our mishnah follows Rabbi Meir, who holds that we do not say "mikelal lav atah shomea hen" and we do not draw that inference. And since he did not explicitly say that the food he was accustomed to eat should become forbidden, it does not become forbidden.
Shevuah formulations that do take effect:
"Shevuah lo ochal lach" - he swore that he would not eat, and the shevuah takes effect.
"Shevuah she'ochal lach" - at first glance this sounds like a person confirming that he will indeed eat, so in what way would anything become forbidden? The Gemara establishes the case as one in which his friend pressed him to eat in his home, and he answered again and again, "I am not going to eat, I am not going to eat," and finally said: "Shevuah she'ochal lach." From the context, and from his repeated refusal, it is clear that he means the negative, and that he took that very idea and expressed it in the language of a shevuah. For that reason he is forbidden to eat.
Some explained the Gemara as holding that "she'ochal lach" means "that I will not eat," and accordingly they established the case as: "Lo shevuah lo ochal lach" - I am not making a shevuah about what I do not eat of yours. The inference is that it should be considered a shevuah about the food he does eat, and by this he forbids it upon himself. And although we said that our mishnah follows Rabbi Meir, who does not derive "mikelal hen atah shomea lav," that applies only to nedarim; regarding shevuos, which are more stringent, even Rabbi Meir agrees that we do derive "mikelal hen atah shomea lav." Therefore, from the statement "Lo shevuah lo ochal lach" one hears "Shevuah she'ochal lach," meaning that he swore that he would eat, and therefore it is forbidden.
"Zeh chomer bishevuos mibanedarim" - this is a stringency in shevuos over nedarim:
On the simple reading, it seems that the mishnah is pointing to the list immediately preceding it: the shevuah cases just listed do take effect and become forbidden, as opposed to the neder cases at the beginning of the mishnah, which do not take effect at all. That, seemingly, is the stringency of shevuos over nedarim.
The Gemara, however, rejects this understanding: the word "chomer" indicates that there is some level of severity in nedarim as well, and here that is not the case. Those neder statements at the beginning of the mishnah are entirely permitted, and are not forbidden at all, not even partially; whereas the word "chomer" implies that nedarim too render things forbidden, only that their severity is not equal.
The Gemara therefore explains that this statement refers back to the previous mishnah. There it was taught that one who says "konam" in the language of a neder about an action - that he will not sleep or that he will not speak - is using a formulation that does not fit a neder, but only a shevuah; and we said that he transgresses "lo yachel devaro" on a rabbinic level. This, then, is the stringency of shevuos over nedarim: one who forbids an action upon himself in the language of a neder transgresses only a rabbinic prohibition, whereas one who swears not to perform a particular action - his shevuah is a full shevuah.
"Vechomer binedarim mishevuah" - and a stringency in nedarim over a shevuah:
Where do we find that nedarim are more stringent than a shevuah? "Amar konam sukkah she'ani oseh, lulav she'ani notel, tefillin she'ani meniach" - one who forbids upon himself, like a korban, the sukkah, the lulav or the tefillin: with a neder it takes effect. Even though in practice it will turn out that he cannot fulfill the mitzvah, he is not saying directly that he cannot fulfill it, nor is he forbidding the action upon himself; rather, he establishes that the sukkah is something from which he may not derive benefit, and consequently he cannot sit in it, since the sukkah is forbidden to him. In these cases, with nedarim the declaration takes effect and the item becomes forbidden.
With a shevuah, by contrast, it is permitted, since a person cannot say "I will not put on tefillin" - the Torah obligated him to put on tefillin. To forbid the tefillin upon himself as an object is within his power, but not to make a statement that he will not put them on. This is because "ein nishba'in levatel es hamitzvos": a person cannot make a shevuah to transgress the mitzvos, and that is precisely what he would be doing by forbidding the act itself upon himself.
In summary: in this mishnah we learned neder formulations that do not take effect - "Korban lo ochal lach" (a prohibition on something he does not eat anyway), "Korban she'ochal lach" (which sounds like an oath by the life of the korban), and "Lo korban she'ochal lach" (regarding which we do not derive "mikelal lav atah shomea hen" according to Rabbi Meir); and, in contrast, shevuah formulations that do take effect, among them "Shevuah she'ochal lach" by force of the context, and "Lo shevuah lo ochal lach" by force of "mikelal hen atah shomea lav," which even Rabbi Meir accepts regarding shevuos. We also examined the two sides of the comparison: the stringency of shevuos over nedarim - that one who forbids an action upon himself in the language of a neder transgresses only rabbinically; and the stringency of nedarim over shevuos - that with a neder one can forbid upon himself the objects of a mitzvah, whereas with a shevuah one may not swear to nullify the mitzvos.