יש אומרים שברכת המזון טעונה כוס אפילו ביחיד וצריך לחזור עליו ולא יאכל אם אין לו כוס לברך עליו אם הוא מצפה ואפשר שיהיה לו אפי' אם צריך לעבור זמן אכילה אחת ולפי זה אם שני' אוכלים יחד צריך לקחת כל אחד כוס לברכת המזון ויש אומרי' שאינה טעונה כוס אלא בשלשה ויש אומרים שאינה טעונה כוס כלל אפילו בשלשה: הגה ומ"מ מצוה מן המובחר לברך על הכוס: [ר"ן פ' ערבי פסחים]:

Some say that Birkat Hamazon requires a cup even when one is alone, and one must wait for it and not eat if he does not have a cup to bless over, if he anticipates and it is possible that he will have one—even if this means passing the time of one eating period. According to this, if two people eat together, each must take a cup for Birkat Hamazon. And some say it is only required when there are three, and some say it is not required at all, even with three: Rema: nevertheless, it is an optimal mitzvah to bless over a cup (Ran, chapter Arvei Pesaḥim).

The cup for Birkat Hamazon must be of wine and not of other beverages, even if he established his meal over them. And if wine is not available in that place, and beer or other drinks are considered ḥemar medina (important local beverage), one may bless over them, except for water: Rema: regarding the custom in these countries to bless over beer, one should not protest, since there are opinions that a cup is not required at all, and furthermore, the primary ḥemar medina is beer, and people establish their meals upon it. And even though wine is found in the city, it is still not considered readily available for this purpose, as it is expensive and not feasible to purchase wine for every meal to bless over. Nonetheless, it is an optimal mitzvah to bless over wine (Darkhei Moshe). Some are meticulous, when blessing alone over wine, not to hold the cup in their hand but to place it on the table before them, and this practice is proper according to the Kabbalah (Beit Yosef)

The cup must not be defective; if someone drank from it, it is defective. But if he poured from it into his hand or another vessel, it is not an issue. Even if he drank from the jug or a small barrel, it is considered defective. But if he drank from a large wooden barrel, one need not be particular. There is an opinion that even defective water is invalid to mix into the cup for a blessing.

If the cups of those who were lounging were unfit, it is necessary to put from the cup of blessing into them. And there is someone who says it is not necessary.

If wine from a defective cup was poured back into the wine jug, the wine in the jug remains valid, because each bit that was poured in becomes nullified as it mixes in (gradually and insignificantly).

People are able to fix an unfit cup by adding a little bit of wine; and even adding water to the cup can it be fixed.

In a pressing moment, we bless upon an unfit cup [of wine]