Gemara

Rav Yeiva the elder was serving Rav Beruna and Rav Chananel, Talmidim of Rav, at a meal. They told him 'Hav Lan u'Nevarech' (give us a cup of wine, and we will bless Birkas ha'Mazon).' They reconsidered and said 'give to us and we will drink.'

Rav Yeiva: Rav says that once you said 'Hav Lan u'Nevarech', you may not drink!

Similarly, if one covered the blood of a Chayah, another blessing is needed for covering the blood of whatever else he will slaughter.

Pesachim 103a: Rava blessed Borei Peri ha'Gafen on the first cup, and again on the cup of Birkas ha'Mazon and drank it.

Rav Yakov bar Aba: Granted, in the Reish Galusa's house we blessed a second time because we were unsure whether we will receive wine. Here, the cup is in front of us, and we plan to have it!

Rava: I did like Talmidim of Rav (after they accepted the words of Rav Yeiva).

Berachos 42a: Rav Papa and Rav Huna brei d'Rav Nasan were served more food after the meal. Rav Papa ate.

Bei Rav Huna: Don't you hold that after the meal, one may not eat?

Rav Papa: That is only if the bread was taken away.

Rava and R. Zeira ate at the Reish Galusa's house. After the serving tray was removed, more portions were sent to them. Rava ate. R. Zeira did not, and asked Rava why he ate.

i. Rava: We intend to eat more if the Reish Galusa will serve more to us (even after our tray was removed).

The Halachah does not follow these teachings. Rather, it is like R. Chiya bar Ashi, who said that one must bless right after washing.

Rishonim

Rif (Pesachim 21a): Once they said that they will bless, they diverted their minds from drinking. Therefore, if they want to drink, they must bless Borei Peri ha'Gafen.

Ran (DH Itsar): Some say that after saying 'Hav Lan u'Nevarech', they may not eat before blessing Birkas ha'Mazon.

Rif (Berachos 30a): Taking away the bread does not forbid eating more. Rather, the Halachah follows R. Chiya bar Ashi, who said that Berachah must immediately follow Netilas Yadayim.

Rambam (Hilchos Berachos 4:7): If one decided not to eat or drink, and afterwards reconsidered to eat or drink, even though he did not change his place, he blesses again. If he did not decide (to stop eating or drinking), rather, he intended to eat or drink again, even if he paused the entire day, he need not bless again.

Rosh (Berachos 6:31): As long as one did not wash for Birkas ha'Mazon, he may eat. According to Rashi and the Rashbam, saying 'Hav Lan u'Nevarech' is like washing. Some distinguish between eating and drinking. This is unreasonable.

Rashi (Chulin 86b DH Kivan d'Amar): Saying 'Hav Lan u'Nevarech' shows that he finished eating. He may not drink without a Berachah Rishonah, for it does not join with his first meal.

Rosh (Chulin 6:6): Rashi and R. Chananel explain that they may not drink without Borei Peri ha'Gafen, but if they bless, they may drink before Birkas ha'Mazon. Just like 'Hav Lan u'Nevarech' is an end (of the meal) and he needs a new Berachah to eat more, also covering the blood is an end, and he must bless again on Shechitah. Some say that they may not drink before Birkas ha'Mazon. This is wrong. This is unlike what we prove from the episode! Also, we bring this episode to support blessing on every cup (Pesachim 103a) and on every slice (Chulin 107b). He need not bless Birkas ha'Mazon every time, rather, ha'Motzi. This proves that Hesech ha'Da'as obligates a new Berachah Rishonah. It does not obligate blessing now on what he already ate. One may bless Birkas ha'Mazon once for two meals. We do not find concern for a Hefsek between an action and a Berachah Acharonah, only between an action and a Berachah Rishonah, e.g. between Netilas Yadayim and ha'Motzi. After eating, as long as he is satiated due to what he ate, he may bless. Rav Yeiva said 'you may not drink', even though he did not know whether they intended to bless Borei Peri ha'Gafen, to teach them that they must bless.

Rosh (Pesachim 10:7): We hold that 'Hav Lan u'Nevarech' does not obligate Birkas ha'Mazon.

Rosh (10): The Rif explains that saying 'Hav Lan u'Nevarech' shows that they diverted their minds from drinking. Therefore a new Berachah is needed. This is primary. The Rashbam forbids drinking even with a Berachah. The Halachah is, even 'Hav Lan u'Nevarech' is Hesech ha'Da'as to obligate Borei Peri ha'Gafen, and all the more so if one picked up the cup of Birkas ha'Mazon to bless.

Tosfos (Chulin 86b DH Asur): Rashi and R. Chananel explain that if they bless, they may drink before Birkas ha'Mazon. If so, why didn't R. Zeira eat with a Berachah? We can say that they were brought another food with bread, and it was hard for him to wash again.

Note: We must say that Rava ate without (washing and) another Berachah, therefore R. Zeira questioned him.

Poskim

Shulchan Aruch (OC 179:1): If one finished his meal and washed Mayim Acharonim, he may not eat or drink before blessing Birkas ha'Mazon. If he said 'Hav Lan u'Nevarech', this is Hesech ha'Da'as, and he may not drink before blessing first on it.

Beis Yosef (DH u'Mah): The Rif, Ran, R. Yonah and Mordechai all say that after deciding not to eat, one may not drink without Borei Peri ha'Gafen. We follow them. The Tur connotes that even after washing, he may eat and drink before Birkas ha'Mazon if he says a Berachah Rishonah. This is wrong. The Rosh connotes that he may not eat until blessing Birkas ha'Mazon. This is why he says that according to Rashi and the Rashbam, saying 'Hav Lan u'Nevarech' is like washing. The other Meforshim hold that 'Hav Lan u'Nevarech' does not obligate Birkas ha'Mazon on what he already ate, but Netilas Yadayim does. It seems that all forbid eating after Netilas Yadayim until blessing Birkas ha'Mazon. Also R. Yonah forbids drinking, even with a Berachah (Rishonah). It is not due to Hesech ha'Da'as, for this does not mandate a Berachah Acharonah on what he ate. Rather, since we hold that Berachah must be right after washing, once he washes to be ready to bless, it is as if he began blessing, and he may not interrupt to eat or drink at all. Saying 'Hav Lan u'Nevarech' is different. Then, only Hesech ha'Da'as applies. Therefore, it suffices to make a Berachah Rishonah on what he wants to eat. Therefore, one may not interrupt between Mayim Acharonim and Birkas ha'Mazon even with conversation. All agree that 'Berachah must be right after washing' applies to Mayim Acharonim. Even the Rosh, who says that it applies to Mayim Rishonim, does not exclude Mayim Acharonim. The Tur (166) says that the Rosh would not interrupt or talk even after Mayim Rishonim.

Taz (1): The Shulchan Aruch distinguishes between Mayim Acharonim and 'Hav Lan u'Nevarech.' In the latter case, one may bless ha'Motzi (and eat more) without Birkas ha'Mazon. Also in the Beis Yosef, he brought the Rosh to challenge the Tur, who permits to make a Berachah Rishonah and eat after Netilas Yadayim. This is astounding. Tosfos says 'one may not eat without blessing after Mayim Acharonim, and the same applies to 'Hav Lan u'Nevarech.' The Rosh follows Tosfos! The Rosh just teaches that Rashi and the Rashbam require Birkas ha'Mazon after 'Hav Lan u'Nevarech.' He did not intend to rule like them! He relies on what he wrote in Chulin, that the Halachah does not follow them. Rather, it suffices to bless ha'Motzi. The Rosh just teaches that 'Hav Lan u'Nevarech' and Netilas Yadayim are the same. According to the other Poskim, a Berachah Rishonah suffices after either of them. The Rosh equates them, like the Tur says. R. Yonah distinguishes them due to '(Berachah must be) right after (washing)', but not due to Hefsek. Surely, after Netilas Yadayim one may make a Berachah Rishonah and eat. It was mere water, not Mayim Acharonim. There is no concern for Hesech ha'Da'as. The Tur agrees; he must wash again before Birkas ha'Mazon. The Beis Yosef says that it is as if he began blessing. This is astounding. There is no concern for Hash-m's name l'Vatalah, for he did not say Hash-m's name! Also the Rema (197:1) says that Netilas Yadayim is like 'Hav Lan u'Nevarech'. It suffices to wash again before Birkas ha'Mazon. One need not be more stringent than this. In Pesachim, the Rosh says 'even though they washed and they are forbidden to eat, we hold that 'Hav Lan u'Nevarech' does not obligate Birkas ha'Mazon.'

Magen Avraham (2): Kitzur Piskei ha'Rosh (the Remazim) explicitly permits to bless and eat after Mayim Acharonim. How can the Beis Yosef says that this is unlike his (longer) Pesakim?!

Machatzis ha'Shekel: Even though the Tur wrote the Remazim, and this is how he explained the Rosh, the Magen Avraham asks as follows. It is feasible that the Tur would overlook something that the Rosh wrote, but he wrote the Remazim with Piskei ha'Rosh in front of him. How could he write the opposite of what is in front of him?!

Magen Avraham (2): The Rosh (Pesachim 10:10) says that one may bless and eat after 'Hav Lan u'Nevarech', and all the more so after taking the cup to bless Birkas ha'Mazon. He takes the cup after Mayim Acharonim! The Bach wrote that a Ba'al Nefesh should be stringent to bless (Birkas ha'Mazon) right after Mayim Acharonim.

Gra (DH Ad): R. Yonah holds like the Beis Yosef. In Berachos and Pesachim Rashi forbids eating after Hav Lan u'Nevarech, but in Chulin he retracted, and all Poskim hold like this.

Magen Avraham (1): I say that one may talk after Mayim Acharonim. In Siman 183:6 we forbid after one was given the cup of Birkas ha'Mazon.

Mishnah Berurah (1): Also, one must be careful not to interrupt with talk, even a little, for Berachah must be right after washing. For this reason, one should not delay a lot after washing, and surely not engage in something. If necessary, one may interrupt for an urgent need. B'Di'eved, if he interrupted, he washes again, so the Berachah will be right after washing.

Mishnah Berurah (2): The Acharonim are lenient to say ha'Motzi and eat after Netilas Yadayim, just he must wash Mayim Acharonim again afterwards. Some say that l'Chatchilah one should be stringent. The leniency is only to eat, but not to speak or engage in something else.

Kaf ha'Chayim (4): Even if he is sure that he guarded his hands, it does not help.

Bi'ur Halachah (DH Ad): Surely one should not be lenient to eat after Netilas Yadayim, for many great Rishonim are stringent, including Rashi, the Rashbam, Or Zaru'a, the Rambam, Ra'avad, and R. Yonah. The Rashba holds that once he prepared himself to bless, it does not help to wash again. The Ro'oh forbids even if he will bless ha'Motzi, for Netilas Yadayim is like accepting on himself to bless Birkas ha'Mazon. A minority of these Rishonim are stringent for this reason even about 'Hav Lan u'Nevarech', and do not allow to say a Berachah Rishonah and eat.

Mishnah Berurah (3): When they bless without a cup, saying 'Hav Lan u'Nevarech' is Hesech ha'Da'as, like 'Hav Lan u'Nevarech.' Similarly, if one explicitly decided not to drink more, and later he wants to drink, he must bless on the drink.

Kaf ha'Chayim (2): L'Chatchilah, whether he said 'Hav Lan u'Nevarech' or washed Mayim Acharonim, he should not eat or drink more. If he must, he blesses a Berachah Rishonah. However, if he only said 'Hav Lan u'Nevarech' and now he wants to eat, he only thinks the Berachah.

Kaf ha'Chayim (3): Some say that mixing the cup of Birkas ha'Mazon is like saying 'Hav Lan u'Nevarech'. If one normally eats from the slice of ha'Motzi at the end of the meal, this is not a Hesech ha'Da'as.