Kiddushin, Chapter Three, Mishnah Five
The first section of this mishnah teaches that if a man was mistaken about some part of a womans identity before he betrothed her but she wasnt the one who mislead him into making that mistake, the betrothal is still valid.
The second two sections of the mishnah deal with a man who tries to betroth a woman who could not currently be betrothed to him. The question is whether the betrothal can become valid at a later point without its being performed again.
1) If he betroths a woman and then declares, I thought that she was a priests daughter, and behold she is [the daughter of] a Levite or a Levites daughter whereas she is [the daughter of] a priest;
a) Poor, whereas she is wealthy, or wealthy, whereas she is poor, she is betrothed, since she did not deceive him.
2) If he says to a woman, Behold, you are betrothed to me after I convert, or after you convert,
a) After I am freed from slavery, or after you are freed from slavery;
b) After your husband dies or, after your sister dies;
c) After your yavam performs halizah for you; she is not betrothed.
3) Similarly, if he says to his friend, If your wife gives birth to a female, behold she is betrothed to me, she is not betrothed.
a) [If his wife is pregnant, and her fetus is discernible, his words are valid, and if she bears a female, she is betrothed.]
Section one: In this case the man made a mistake with regard to the womans identity, but she didnt cause the mistake. The betrothal is therefore valid. After all, were this grounds to annul a betrothal a man could always annul his betrothal. He could always say that he thought something was true and it turned out to be untrue and there would be no way of falsifying his claim.
Section two: The general principle in this section and in the next is that if the betrothal is invalid now, it cannot become valid later on through an act performed now. There are five things that prevent the betrothal from being possible at this point.
1) Either the man or woman is not Jewish.
2) Either the man or woman is a slave.
3) The woman is married.
4) The man is married to the womans sister.
5) The woman is awaiting halitzah (release from levirate marriage).
In all of these cases, the betrothal cannot currently occur. Therefore, even if he says that the betrothal should only become valid when their statuses change such that betrothal would become possible, the current act of betrothal is invalid.
Section three: In this section too the man is attempting to betroth someone in the future to whom he could not be currently betrothed. Here he couldnt betroth her now because she doesnt even exist. She hasnt even yet been conceived. This guy is really trying to get his bid in early! Again, the mishnah rules that his kiddushin are invalid.
The last section of the mishnah is an addition to the mishnah (meaning it is missing in mishnaic manuscripts). It is clause from the Talmud that somehow crept in and it changes the meaning of the mishnah, for here we have an example where kiddushin that could not currently occur can be currently contracted. According to this clause, as long as it is already recognized that the woman is pregnant, the betrothal is valid and if a girl is born she is betrothed to that man. In order to make this clause match the remainder of the mishnah, the Rambam explains that he must betroth her again when she is born.