Gemara

(Rav Huna): Both a virgin and a widow require a Berachah.

Contradiction: Rav Huna said that a widow does not require a Berachah!

Answer: For a Bachur (one who was not married before), we must bless, but not for a widower who marries a widow.

Objection: A Beraisa learns from Boaz and Rus, a widower and a widow, that we need 10 men for Birkas Chasanim!

Answer: Rav Huna meant that a widow does not need seven days of Berachah. One day is required.

Question (Beraisa): Chachamim deliberated how to help Benos Yisrael, so a Chasan will rejoice three days (with a widow).

If he is a Bachur, seven days are required. If he is a widower, only one is required!

Answer #1: A widower requires one day of Berachah and three days of Simchah.

Answer #2: A Bachur requires seven days of Berachah and three days of Simchah.

Question (Beraisa): We bless seven days for a virgin, and one day for a widow.

Suggestion: This applies even to a Bachur who married a widow!

Answer: No, it applies only to a widower.

Question: If you hold that for a Bachur we bless seven days, it should say that we bless seven days for a virgin or for a Bachur who married a widow, and one day for a widow (who marries a widower)!

Answer: The Tana taught a clear-cut law, that every virgin has seven days of Berachah, and every widow has no less than one.

7b (Beraisa): We bless Birkas Chasanim all seven days with 10 people.

(Rav Yehudah): This is when there are Panim Chodoshos (new people) at the meal.

8a: Rav Ashi came to Rav Kahana's house. The first day he made all the Berachos.

After the first day, if there were Panim Chodoshos he would bless all the Berachos. If not, they merely increase Simchah, so he blessed only "...sheha'Simchah bi'M'ono (that joy is in His dwelling)", and "Asher Bora Sason (Who created joy...)"

From seven days after the wedding until 30, whether or not the guests were invited due to the wedding, he blessed "...sheha'Simchah bi'M'ono".

After 30 days, if the guests were invited due to the wedding, he blessed "sheha'Simchah bi'M'ono"; if not, not.

Rishonim

The Rif and Rosh (1:13) bring our Gemara.

Rif: Asher Bora begins with Baruch because we bless it even when there are not Panim Chodoshos. Therefore, it is a Berachah unto itself (it is not merely one of a series of Berachos).

Ran (DH Achas): The two answers (that distinguish Berachah from Simchah) do not argue. When he is a widower, there are less days of Berachah (one) than of Simchah (three). When he is a Bachur, there are more days of Berachah (seven) than of Simchah (three)! This is because Berachah is for his joy. If this is his first marriage, it is great. Simchah, i.e. neglecting work to rejoice with her, was enacted for her. A widow need not be enticed so much to marry. The Rambam (Ishus 10:12) holds like this. Some say that the two answers argue, and we follow the latter answer. When he is a Bachur, there are seven days of Berachah and of Simchah. It is unreasonable that he goes to work and we bless Birkas Chasanim. This is the custom.

Ran (DH v'Hu): Also Sheha'Kol Bora li'Chvodo and Yotzer ha'Adam are not part of the series. They do not discuss marriage. They are merely to honor the people who came, and the initial formation of man, respectively. We bless sheha'Simchah bi'M'ono at the beginning of the Zimun.

Ran (DH Mevarech): The Ramban infers from Levi and Rav Asi, who blessed (only) five and six Berachos, that sheha'Simchah bi'M'ono is not Birkas Chasanim, and therefore it does not require 10. I do not understand. It is not a Berachah unto itself, it is merely an addition to Birkas ha'Mazon! We infer that the Ramban considers Asher Bora among the five and six that require 10. The Beraisa said 'we bless Birkas Chasanim with 10 all seven days', which connotes that all the Berachos are the same. The Ramban is forced to say this is when there are Panim Chodoshos. This is difficult. Rather, just like sheha'Simchah bi'M'ono is not part of Birkas Chasanim (we would say it at a Bris, if not for the pain of the baby), also Asher Bora. Also Asher Bora does not need 10.

Ran (ibid.): The Ramban infers from 'if there were no Panim Chodoshos... they merely increase Simchah' that they invited others. If not, we do not bless even sheha'Simchah bi'M'ono. This is wrong. The Gemara said 'seven days for Berachah', i.e. even if he eats with his household.

Rambam (Hilchos Berachos 2:9): We bless Birkas Chasanim (Asher Bora) in Beis Chasanim after the four Berachos (of Birkas ha'Mazon) at every meal eaten. If both of them were married before, we do so only on the first day. If either of them was not married before, we do so all seven days of feasting.

Hagahos Maimoniyos (4): Nowadays that all Simchah has darkened, we never bless these Berachos after seven days.

Rambam (10): This is when those who ate heard the Berachos of Nisu'in at the Nisu'in. If there were Panim Chodoshos who did not hear the Berachos of Nisu'in at the Nisu'in, we bless for them after Birkas ha'Mazon the seven Berachos like we bless at the Nisu'in. This is if there are 10, including the Chasanim.

Rosh (1:13): We do not need 10 to bless "sheha'Simchah bi'M'ono". Some say that Asher Bora requires 10. Others say that we need 10 only to bless all the Berachos, for we learn from Boaz or from "mi'Mekor Yisrael".

Poskim

Shulchan Aruch (EH 62:4): We bless Birkas Chasanim only with 10. We do not need 10 for Asher Bora.

Rema: We do require three.

Shulchan Aruch (5): We bless Birkas Chasanim in Beis Chasanim after Birkas ha'Mazon at every meal eaten there.

Shulchan Aruch (6): If both of them were married before, we bless only on the first day.

Beis Shmuel (5): The Rosh says that the 'first day' refers to the first meal. Even if (the Nisu'in was during the day and) they did not eat until night they bless, for they are like Panim Chodoshos, for they did not yet eat. R. Yerucham understands that it is the first day. Therefore, a widow and widower would not bless at night, for they do not bless after the first day even with Panim Chodoshos! The Rif holds that they bless the entire first day, even without Panim Chodoshos. Whenever there is an argument, we are lenient about Safek Berachos.

Shulchan Aruch (ibid.): If either of them was not married before, we bless all seven days of feasting.

Rema: The seven days begin right after the first Sheva Berachos (at the Nisu'in).

Shulchan Aruch (7): The Berachah we add in Beis Chasanim is Asher Bora.

Rema: Some say that even Asher Bora we add only when others were invited, but not if he eats with his household. This is the custom.

Chelkas Mechokek (8): The Ran holds that we say Asher Bora even when we do not say sheha'Simchah bi'M'ono. Since we do not say Asher Bora when he eats with his household, all the more so sheha'Simchah bi'M'ono.

Bach (YD Sof 391 DH Kosav ha'Roke'ach): We bless Asher Bora and sheha'Simchah bi'M'ono until Bi'as Mitzvah. In Krakao we omit sheha'Simchah bi'M'ono on the second night, even though Bi'as Mitzvah will be later that night, because men are gathered in a room with some women and there are thoughts of sin. (Hash-m has no Simchah where there is Isur.)

Rebuttal (Beis Shmuel 7): The Gemara says that Bi'as Mitzvah of a virgin is Wednesday night, yet we bless Sheva Berachos all seven days!

Shulchan Aruch (ibid.): This is when those who ate heard the Berachos of Nisu'in at the Nisu'in. If others ate who did not hear the Berachos of Nisu'in at the Nisu'in, we bless for them after Birkas ha'Mazon the seven Berachos like we bless at the Nisu'in. This is if there are 10, including the Chasan. Some say that even if they heard the Berachos at the Chupah if they did not eat there until now they are called Panim Chodoshos. This is the custom.

Yavi'a Omer (5:EH 7): Sheva Berachos is (only) in the house of the Chasan. (Note: Ashkenazi and many Sefardi Poskim disagree.)