Gemara

Question (Rav Acha brei d'Rava): We answered every question against Rava and Abaye. What do they argue about?

Answer: They argue about whether Shinuy (a change in a stolen object) acquires.

Berachos 27b - Question: Must one indeed accept Shabbos before praying Ma'ariv of Shabbos?!

(R. Avin): Rebbi once prayed Ma'ariv of Shabbos on Erev Shabbos, entered the bathhouse (which is forbidden on Shabbos) and taught to us!

Answer (Rava): Rebbi entered the bathhouse to sweat, before Chachamim prohibited this on Shabbos.

Rishonim

Tosfos (6a DH v'Hashta): Why didn't the Gemara say that Abaye and Rava argue about one who swore not to divorce his wife, and then divorced her? I answer that since he himself invented the Isur, in such a case Rava agrees that if he transgressed, it takes effect.

Shitah Mekubetzes (Beitzah Sof 30a): The Gemara says that women who eat until the last moment before Yom Kipur transgress. Tosfos derived from here that Tosefes Yom Kipur has a Shi'ur. The Ri says that it is about half an hour, and the same applies to Shabbos. If so, why did we learn that one may not light the Neros when it is Safek dark? This implies that until it is a Safek, one may light? It should be forbidden due to Tosefes Shabbos! We can say that during Tosefes Shabbos one may tell a Nochri to light, for this is Shevus. When it is Safek dark, one may not even tell a Nochri.

Poskim

Shulchan Aruch (OC 261:2): Some say that one must add some time that is Vadai from Chol to Kodesh.

Mishnah Berurah (19): The Tosefes must be before Bein ha'Shemashos, which is Safek dark. We do not need a verse to include it!

Kaf ha'Chayim (22,27): The Levush says that one who accepted Shabbos before dark can permit this, for it is like a mere vow. Mishbetzos Zahav says that this is only if he accepted between Plag ha'Minchah and Shki'ah, which is mid'Rabanan. If he accepted after Shki'ah, it is mid'Oraisa and he cannot permit it. Eliyahu Rabah was unsure whether it can be permitted at all. It seems that the Magen Avraham holds that it does not. The Ari Zal holds that if one accepted Shabbos through saying Bo'i Kalah he received (extra) Nefesh, Ru'ach, Neshamah, Chayah and Yechidah (the five levels of soul), so he cannot retract. Also, he may not eat until saying Kidush. If he said merely 'I accept Shabbos', he can retract.

Shulchan Aruch (4): After answering Borchu, even if it is still day, one may not be Me'arev or wrap, for he accepted Shabbos on himself. For us, saying Mizmor Shir l'Yom ha'Shabbos is like Borchu was for them.

Chacham Tzvi (11): Women had the custom to immerse in hot water on Shabbos like during the week. A Chacham permitted them to immerse during the seventh clean day. This is wrong. It is forbidden to immerse in hot water on Shabbos, and if one does so, we call him a transgressor. However, a woman may not immerse on the seventh day. She must immerse in cold water, or postpone the Tevilah until Motza'ei Shabbos, or blunt the chill of the water, but not to make it what people call hot. Alternatively, she may immerse Bein ha'Shemashos. All decrees mid'Rabanan do not apply Bein ha'Shemashos. However, this is not simple. The Beis Yosef rules like R. Shmarayah, who initially permitted to be Me'arev after praying Ma'ariv, but retracted. A Tosefta says that at the Teki'os (sounding of the Shofar at the start of Shabbos), even if he is holding a kettle, he must put it down (he may not wrap it to stay warm). They enacted the Teki'os in order that people will remove pots from the fire and wrap them. This applies every Erev Shabbos. One who did not wrap at the proper time was negligent. He may not wrap afterwards. If not, the enactment is Batel. Others will see that he wrapped after the Teki'os, and they will do similarly, and the matter will spread. Tevilah in hot water on Shabbos is a sporadic matter. It is permitted even after the Teki'os, as long as it is not Vadai night.

Question: In Berachos, the Gemara answered that the episode in which Rebbi entered a bathhouse after praying Ma'ariv was before Chachamim decreed. Why didn't it answer that their decrees do not apply Bein ha'Shemashos?

Answer (Chacham Tzvi): The Heter is only for a Mitzvah or pressed circumstances, like the Rambam and other Poskim say. The Gemara knew that these did not apply. We find that after, he had time to teach the Talmidim! If there were any Isur, he should have bathed before praying. The Maharshal (Teshuvah 46) connotes that one may do Shevus even after praying.

Taz (OC 600:2): Once, due to Ones a Tzibur did not get a Shofar until Bein ha'Shemashos of between the second day of Rosh Hashanah and Shabbos, after they already prayed Ma'ariv. A Mishnah permits making Eruv Chatzeros Bein ha'Shemashos. The Mordechai (Shabbos 290) says that the Ri and R. Chananel permit also Eruv Techumim. R. Yoel permits even after praying Ma'ariv, if it is still day. Answering Borchu is like Teki'ah, and it is permitted when it is Safek dark, after the Teki'os. Some say that one who accepted Shabbos is different (this is more stringent). R. Shmarayah brought a proof from the Tosefta, which obligates putting down a pot at the Teki'ah, even though Hatmanah is permitted when it is Safek dark. This shows that acceptance is more stringent. I disagree. The Tosefta forbids putting the pot on a fire, but not Hatmanah! The Beis Yosef (Reish Siman 342) forbids Shevus only during Shabbos itself, but not during Tosefes. Even those who are more stringent about acceptance than Safek dark say so only about Reshus. They would agree here (regarding Shofar) that acceptance is lighter regarding a Mitzvah. He did not intend to accept if he will lose a Mitzvah! It is like a mistaken acceptance, like we say about one who accepted because it was cloudy (and he thought that it was dark - 263:14). Even if he knew that a Shofar will come later, acceptance is no greater than an oath, and one may not swear to uproot a Mitzvah. We find (Beis Yosef YD 262, citing Semag) that the day for Bris Milah does not depend on whether the baby was born before or after they prayed Ma'ariv. The same applies if Milah was not done on day eight until they prayed Ma'ariv. It does not affect whether one may still do Bris Milah.

Chacham Tzvi (DH Acharei): The Taz erred regarding putting the pot down after the Teki'ah. We permit Hatmanah after the last of the first three Teki'os. The Tosefta discusses after the last of the last three Teki'os.

Machazik Berachah (261:5 DH va'Tabet): Regarding whether one may be Me'arev, after acceptance is more stringent than Bein ha'Shemashos. Do not say that this is because Tosefes Shabbos is an Aseh, which is stronger than and overrides Lavin and surely a Safek Lav. Some say that the punishment for an Aseh is lighter, e.g. is one must eat Isur, it is better to eat an Isur Aseh (Tosefes Yom ha'Kipurim 83a DH Tevel). Also, Kares is more stringent than a Stam Lav, and the Aseh of Tosefes is weaker than a Stam Aseh, for it is not explicit in the Torah. An Aseh that one made on his own (e.g. a vow) is weaker than a regular Aseh. Bein ha'Shemashos is a Safek Lav and Aseh.

Machazik Berachah: It seems that the Shitah Mekubetzes holds like R. Yoel, that Bein ha'Shemashos is more stringent than acceptance of Shabbos. Even though we could say that he discusses one who did not accept Shabbos, just Tosefes Shabbos comes automatically half an hour before, in any case he clearly holds like R. Yoel that Tosefes Shabbos has a Shi'ur. Just like Shevus does not apply then, the same applies to one who accepted during the day. (According to this, - PF) surely, also Bein ha'Shemashos Shevus is permitted even if he accepted. However, if an entire Tzibur accepted Shabbos, perhaps even this opinion would forbid Shevus, for not everyone knows the Halachos, and people might come to belittle Shabbos. This resolves the Tosefta. The Poskim Rishonim did not specify; this connotes they hold like R. Yoel. An individual may rely on this (like the Mechaber connotes that in 393:2), but not if a Tzibur accepted, like the Mechaber wrote (Halachah 4).

Mishnah Berurah (31): The Magen Avraham says that nowadays the custom is to say Mizmor Shir, and even so we do all Melachos until Borchu (if it is still Bein ha'Shemashos). This is because they do not intend to accept Shabbos through saying this. Nowadays, the custom is that the Tzibur accepts through Mizmor Shir and all Melachos are forbidden, even if much time remains in the day. Derech Chachmah says that similarly, where they conclude Lecha Dodi with "Bo'i Kalah", this is Kabalas Shabbos.

Shulchan Aruch (393:2): One may make Eruv Chatzeros Bein ha'Shemashos, even if he already accepted Shabbos. Some forbid of he accepted Tosefes Shabbos.

Mishnah Berurah (12): The Gra leans to rule like the stringent opinion.

Question: May women ride on a wagon, which a Nochri conducts, to immerse after lighting Neros Shabbos? Even though they accepted Shabbos, perhaps we permit Shevus for the sake of a Mitzvah.

Answer (Rav Pe'alim OC 49): We permit Shevus for the sake of a Mitzvah or in pressed circumstances when it is Safek dark without acceptance. If one accepted, R. Shmarayah forbids, and R. Yoel permits. Machazik Berachah concludes to be lenient, but if a Tzibur accepted, they should be stringent. Perhaps even R. Shmarayah would be lenient about an individual's acceptance. The Chida would permit women to ride after accepting Shabbos, but I am hesitant, because several Acharonim disagree with the Chida. Also, perhaps Rabanan permitted only b'Di'eved. Perhaps even R. Yoel and the Chida would not permit to light the Neros and accept Shabbos with intent to ride afterwards. Also, they were lenient only when the Mitzvah can be done only through the Shevus. Here, the women can walk. We do not permit merely to avoid toil. And even if she could go only if she rides, perhaps she must stipulate that she does not accept until they say Borchu in the Beis ha'Keneses. The Mechaber (263:10) brought first the opinion that a Tanai helps (which shows that he favors it). The Rema rules like it. One should stipulate only in a case of need. Therefore, she may not ride without stipulating. She must be sure to get off the wagon before the Tzibur says Borchu. If different Minyanim pray one after the other, she can stipulate to accept after the last one says Borchu.