Gemara

Mishnah - R. Eliezer: One must eat 14 meals in the Sukah, one every day and one every night;

Chachamim say, there is no quota except for the first night

R. Eliezer also said, if one did not eat the first night he should be Mashlim (complete the quota) on the night of Shemini Atzeres.

Chachamim say, there is no compensation - "Me'uvas Lo Yuchal Liskon..."

R. Eliezer learns from "Teshvu" k'Ein Taduru - just like one normally eats one meal during the day and one at night, so he must do in the Sukah;

Chachamim say, just like one normally eats only if he wants to, the same applies to eating in the Sukah.

Question: If so, even the first night it should be optional!

Answer (R. Yochanan): We learn from a Gezerah Shavah "Chamishah Asar - Chamishah Asar" from Pesach. Just like one must eat Matzah the first night, and afterwards it is optional, the same applies to eating in the Sukah.

Mishnah - R. Eliezer: one can be Mashlim (on Shemini Atzeres).

Question: R. Eliezer said that one must eat every day and every night in the Sukah! (How can he be Mashlim when there is no Mitzvah of Sukah?)

Answer (Bira): R. Eliezer retracted.

Question: With what is he Mashlim?

It cannot be with bread. This is a normal meal (nothing indicates that it is to compensate)!

Answer: He is Mashlim with kinds of Targima (this will be defined).

Support (Beraisa): If one was Mashlim with kinds of Targima he was Yotzei.

Berachos 49b - Shmuel: If one forgot to mention Rosh Chodesh in Shemoneh Esre, he must repeat it. If one forgot Rosh Chodesh in Birkas ha'Mazon, he does not repeat it.

Question (Rav Avin): Why is Shemoneh Esre different than Birkas ha'Mazon?

Answer (Rav Nachman): Shemoneh Esre is an obligation, therefore one must repeat it. Birkas ha'Mazon is not an obligation, i.e. one need not eat, therefore he need not repeat it.

Question (Rav Avin): According to this, on Shabbos and Yom Tov, when one must eat, he should have to repeat it!

Answer (Rav Amram): Yes! Rav said so.

Rishonim

Tosfos (DH Chazar): Rashi (DH Chazar) says that R. Eliezer retracted to hold like Chachamim that there is no quota. He holds that if one did not eat in the Sukah on the first night there is compensation. This is wrong. Rather, he retracted to say that the 14 meals need not be in the Sukah.

Rif and Rosh (2:15): Chachamim learn from a Gezerah Shavah that the obligation to eat in the Sukah is only on the first night.

Rambam (Hilchos Sukah 6:7): It is obligatory to eat in the Sukah on the first night. Even if he eats a k'Zayis of bread he was Yotzei. After this it is optional. If he wants to eat a meal, he must eat in the Sukah. If he wants to eat only fruits and parched grain outside the Sukah, he may. This is like eating Matzah on Pesach.

R. Mano'ach: This is why we bless 'Leshev ba'Sukah', and not 'Le'chol ba'Sukah'.

Rosh (ibid.): The Yerushalmi asks (but does not answer) whether or not one must enter Sukos hungry to eat with appetite, like Matzah on Pesach, and whether or not one is Yotzei with kinds of Targima.

Question (Rosh Berachos (7:23) and Ran (DH Masnisin)): The Gemara (Berachos 49b) says that one need not eat on Rosh Chodesh. This refers to bread, for one may not fast on Rosh Chodesh (Ta'anis Sof 10a). It says that on (every) Shabbos and Yom Tov one must eat, i.e. bread. If so, why is a Gezeirah Shavah needed to obligate eating bread in the Sukah on the first night?

Answer #1 (Rosh, ibid.): The Gemara says that one must eat bread on Shabbos and Yom Tov, i.e. the first night of Pesach and Sukos. The obligation to repeat Birkas ha'Mazon to correct an omission of Ya'aleh v'Yavo is only on these two nights.

Objection (Rosh, ibid.): Surely the Gemara discusses the daytime. It says that one goes back if he forgot to mention Rosh Chodesh in Shemoneh Esre, but not in Birkas ha'Mazon. At night, one does not repeat Shemoneh Esre for forgetting Rosh Chodesh.

Answer #2 (Rosh, ibid.): One is obligated to eat bread on every Shabbos and Yom Tov because of Simchas Yom Tov. The Gezeirah Shavah obligates eating in the Sukah on the first night of Sukos, i.e. even if he would normally be exempt from Sukah due to rain.

Answer #3 (Ran, ibid.): Normally, a k'Beitzah suffices for a meal of Shabbos or Yom Tov. The Gezeirah Shavah obligates an amount that requires a Sukah, i.e. more than a k'Beitzah. Some say that the Gezeirah Shavah obligates only a k'Zayis. Even though the rest of Sukos a k'Zayis is Arai and may be eaten outside the Sukah, on the first night the Torah makes even this Keva.

Poskim

Shulchan Aruch (OC 639:3): It is obligatory to eat in the Sukah on the first night. Even if he eats a k'Zayis of bread he was Yotzei.

Mishnah Berurah (21): It is a Safek if one may eat Targima or cake in place of bread. Since the Chiyuv is mid'Oraisa, one must be stringent.

Kaf ha'Chayim (52): If one ate enough cake to be Kove'a Seudah he was Yotzei.

Mishnah Berurah (22): This is if one has only a k'Zayis of bread. If he has more he should fulfill the opinions that require more than a Beitzah. He should eat it within Kdei Achilas Pras.

Kaf ha'Chayim (50): One should not talk after blessing ha'Motzi until eating a k'Beitzah.

Shulchan Aruch (ibid.): After the first night it is optional. If he wants to eat a meal, he must eat in the Sukah. If he wants to eat only fruits and parched grain outside the Sukah, he may, just like eating Matzah on Pesach.

Magen Avraham (10): This refers to Chol ha'Mo'ed. On Shabbos and Yom Tov one must eat at least a k'Zayis of bread, but it need not be in the Sukah.

Mishnah Berurah (23): On every Shabbos and Yom Tov it is better to eat more than a Beitzah to make the meal Keva, but one is Yotzei with a k'Zayis.

Mishnah Berurah (24): The Gra holds that there is an Aseh to eat Matzah on the first night, and the rest of Pesach it is a mere Mitzvah. Perhaps likewise on Sukah it is a Mitzvah to eat in the Sukah every day and to bless 'Leshev ba'Sukah'.

Rema: One should not eat on the first night until it is Vadai night.

Magen Avraham (11): There is no Isur to eat before night. One may make Kidush shortly before dark, and bless ha'Motzi and Leshev ba'Sukah at night. However, since we bless Leshev ba'Sukah before Shehecheyanu, one may not make Kidush until night.

Rema (ibid): One must eat before midnight. One should not eat after noon on Erev Sukos, in order that he will eat in the Sukah with appetite, like eating Matzah.

Question (Magen Avraham 12): On Erev Pesach one may eat until the 10th hour! Perhaps Erev Pesach is more lenient, for then the only permitted bread is Matzah Ashirah, which is not satiating. It is not clear what is the source for this.

Kaf ha'Chayim (60): Many say that it suffices to refrain from eating three hours before night on Erev Sukos. Some say that this applies to other satiating foods, but bread is forbidden after noon.