Yevamos 30

The Gemara says that if a man was married to an Ervah to his brother and a regular woman and Safeik divorce the Ervah, when he dies the regular woman is exempt from Yibum. Since the Ervah has a Chazuka that she's married to the brother, her Tzara has a Chazaka that she's exempt from Yibum. Tosfos points out that even if the Safeik divorce happened before he married the other woman, where I might say that she was never definitely married along with the Ervah so she never had a Chazuka to be a Tzuras Ervah, yet she's exempt from Yibum. Since the Ervah has a Chazuka that she's married to the brother, therefore we extend that Chazuka to her that we consider her a TZuras Ervah. Rabbi Chaim Smulowitz If you ever though that Tosfos was beyond you. Think again. A few more free copies of Gemara and Tosfos: Sukka 2a-11a is still available at tosfos.ecwid.com

Yevamos 29

The Gemara concludes that R' Eliezer doesn't necessary hold that Maamar acquires the Yevama completely. (In the second answer) The reason he holds that he can annuls the Yevama's oath is because we refer to a case where the time that he needs to do Yibum arrived. Although he didn't do it yet, Beis Din obligates him to feed the Yevama. Since she's receiving her support from the Yuvam, when she makes an oath, she relies on the Yuvam to approve it, so it was only made on condition that the Yuvam consents.

Tosfos says that R' Yehusha that argues (and says that anytime there is more than one Yuvam no one can annul her oath) holds even if one gave Maamer, since the other brother can ruin it for him by giving a Get or having relations with her, the Yevama is not confident that he'll eventually marrying her, so she doesn't  make her oath on condition that the Yuvam consents.

R' Akiva (that holds that a Yuvam never can annul an oath) we don't necessary have to explain him like Rashi, that he holds there is no Zika so he's a complete stranger to her since there is no connection. We can says there is really a connection (Zika.) However, since the connection is a lot weaker than an actual wife (as proof, we see that if someone else has relations with the Yevama there is no death penalty)  therefore he's not enough of a husband that the Parsha of Hafaras Nedarim should apply to him

Rabbi Chaim Smulowitz 

If you ever though that Tosfos was beyond you. Think again. A few more free copies of Gemara and Tosfos: Sukka 2a-11a is still available 
at 
tosfos.ecwid.com