Tana'im argue about "I will complete the number of your days":

(Beraisa - R. Akiva): "I will fulfill the number of your days" - these are the years apportioned to a person when he is born. If he merits, they are fulfilled. If not, they are reduced;

Chachamim say, if he merits, they are increased. If not, they are reduced;

Chachamim: It says, "I will add 15 years to your days"!

R. Akiva: The years added were allotted to him from before.

Support (R. Akiva, for himself): It was prophesized "A son will be born to Beis David, his name will be Yoshiyahu". Menasheh (from whom Yoshiyahu descended) was born during Chizkiyahu's 15 'extra' years!

Rejection (Chachamim): The verse says that he will be born to the house of David, but it did not specify that he will descend from Chizkiyahu!

(Mishnah): If his wife died... if his Yevamah died... (he may marry her sister).

(Rav Yosef): This Mishnah is not needed (the Heter is easily derived from a verse).

(Mishnah - R. Gamliel): If a Yavam gave a Get to his Yevamah, a second Get does not take effect (to forbid the giver to the receiver's relatives). A Ma'amar does not take effect after a Ma'amar, nor does Bi'ah take effect after Bi'ah, nor Chalitzah after Chalitzah;

Chachamim say, a Get takes effect after a Get and a Ma'amar takes effect after a Ma'amar, but nothing takes effect after Bi'ah or Chalitzah.

If a Yavam gave a Ma'amar to his Yevamah and then a Get, she needs Chalitzah;

If he gave a Ma'amar then did Chalitzah, she needs a Get;

If he gave a Ma'amar and had Bi'ah, this is proper fulfillment of the Mitzvah;

If he gave a Get and then a Ma'amar, she needs a Get and Chalitzah;

If he gave a Get and had Bi'ah, she needs a Get and Chalitzah;

If he gave a Get and did Chalitzah, nothing is needed after Chalitzah.

If he did Chalitzah, then gave a Ma'amar or a Get or had Bi'ah; or, if he had Bi'ah, then gave a Ma'amar or a Get or did Chalitzah, nothing takes effect after Chalitzah. (We will ask (53b) why the Mishnah didn't also say that nothing takes effect after Bi'ah.).

All this applies when there is one Yavam, whether there is one Yevamah or two.

If he gave a Ma'amar to each Yevamah, each needs a Get, and Chalitzah must be done;

If he gave a Ma'amar to one Yevamah and a Get to the other, the former needs a Get and Chalitzah;

If he gave a Ma'amar to one Yevamah, and had Bi'ah with the other, each needs a Get, and Chalitzah must be done;

If he gave a Ma'amar to one Yevamah and did Chalitzah to the other, the former needs a Get;

If he gave a Get to each, they need Chalitzah;

If he gave a Get to one and had Bi'ah with the other, the latter needs a Get and Chalitzah;

If he gave a Get to one and a Ma'amar to the other, the latter needs a Get and Chalitzah;

If he gave a Get to one and did Chalitzah to the other, nothing takes effect after Chalitzah;

If he did Chalitzah to both, or did Chalitzah to one and gave a Ma'amar or a Get or had Bi'ah with the other, or had Bi'ah with both, or had Bi'ah with one and gave a Ma'amar or a Get or did Chalitzah to the other, nothing takes effect after Chalitzah;

All this applies to one Yavam and two Yevamos, or two Yevamim and one Yevamah.

If he did Chalitzah and gave a Ma'amar or a Get or had Bi'ah, or, if he had Bi'ah and gave a Ma'amar or a Get or did Chalitzah, nothing takes effect after Chalitzah. (Here we return to discuss one Yevam and one Yevamah);

This is whether the Chalitzah was at the beginning, in the middle or at the end.

When Bi'ah is at the beginning, nothing takes effect after it. If Bi'ah is in the middle or at the end, things take effect after it;

R. Nechemyah says, Bi'ah and Chalitzah are similar. Whether they are at the beginning, middle or end, nothing takes effect after them.

(Gemara): They argue only about a Get after a Get or a Ma'amar after a Ma'amar, but all agree that a single Get or Ma'amar takes effect on a Yevamah.

Question: Why was Get enacted?

Answer: A Get permits a married woman; Chalitzah also permits a woman. If people see that Get does not affect a Yevamah, they will think that also Chalitzah does not affect her, and they will come to do Yibum after Chalitzah.

Question: Why was Ma'amar enacted?

Answer: Ma'amar (i.e. money or a document give for Kidushin) acquires a single woman; Bi'ah also acquires. If people see that Ma'amar does not acquire a Yevamah, they will think that also Bi'ah does not acquire, and they will come to do Yibum after Yibum.

Question: Why did Chachamim say that Zikah remains after a Pasul Bi'ah (i.e. a forbidden Yibum)?

Answer - part 1: When Bi'ah is after a Get, this is a decree due to Yibum after Chalitzah (if we would say that no Zikah remains, people would think that it is a proper Yibum);

Answer - part 2: When Bi'ah is after a Ma'amar, this is a decree due to Yibum after Yibum.

Question: Why did Chachamim say that no Zikah remains after a Pasul Chalitzah (i.e. it was forbidden to do Yibum)?

Answer: There is no reason to decree!

Suggestion: We should decree about Chalitzah after a Get due to Chalitzah after Chalitzah.

Rejection: There is no problem if people do extra Chalitzos!

Suggestion: We should decree about Chalitzah after a Ma'amar due to Chalitzah after Yibum (lest people think that after Yibum, the Yavam can dismiss her with Chalitzah alone).

Rejection: In this case, we require a Get for the Ma'amar. Also when Chalitzah is done after Yibum, people will give a Get!