Maseches Kesubos, Chapter 1, Mishnah 2. The mishnah before us establishes the amounts of the kesubah for different categories of women, and the question of in which cases the husband has a claim regarding virginity.
"Besulah kesubasah masayim ve'almanah maneh" - a virgin's kesubah is two hundred and a widow's is a maneh:
"Besulah" - a young woman who is marrying and has never been married before: her kesubah is two hundred zuz, which are two hundred dinar.
"Almanah" - a widow: her kesubah is a maneh, one hundred dinar, half of that amount.
A virgin who is a widow, a divorcee, or a chalutzah from betrothal:
The mishnah goes on to list a woman who has gone through one of these experiences: she lost her husband, she was divorced, or her husband died and she received chalitzah - but all of this only "min ha'eirusin" - from betrothal, that is, from kiddushin, before the marriage was completed through actual nisuin. Since she never came to a full marriage, her presumed status is that she is still a virgin, and therefore she is called a virgin - "u'kesubasan masayim" - and their kesubah is two hundred. Even though she was already married once before, she retains the presumption of virginity, since she never had relations with a man.
Furthermore: "ve'yesh lahen ta'anas besulim" - and they are subject to a claim regarding virginity. If she is found not to be a virgin, the husband may claim against her that his marriage was a mekach ta'us, a marriage entered into on the basis of a false presentation, and therefore she forfeits her right to a kesubah.
The convert, the captive, and the maidservant:
The next case in the mishnah deals with a girl who comes from a non-Jewish background, or from a situation in which the presumption about people of that status is that they had relations through promiscuity and forbidden relations even at a young age. However, a girl who is under the age of three - even if she had relations, she remains a virgin: the physiology is such that she does not lose her virginity, but rather it returns and is renewed, since at such an early stage the act is not considered an act of relations at all.
There are three of them:
The convert - a girl who converted.
The captive - a Jewish girl held in captivity among gentiles, regarding whom there is a presumption that she was forced into relations.
The maidservant - a gentile maidservant, regarding whom there is likewise a presumption that she had relations.
And all three of them left their situation before the age of three, in the language of the mishnah: "shenifdu veshenisgayru veshenishtachreru" - who were redeemed, who converted, and who were freed. The captive was redeemed, the convert converted, and the maidservant was freed - "pechusos mibenos shalosh shanim ve'yom echad" - when they were less than three years and one day old.
Therefore "kesubasan masayim" - their kesubah is two hundred, because they retain the presumption of virginity. Even if there was some act of relations, it does not have the halachic status of relations, and physiologically it is not considered an act that removes her virginity; rather, her virginity returns and is renewed. And since they retain the presumption of virginity, they too "yesh lahen ta'anas besulim" - are subject to a claim regarding virginity. If she is found not to be a virgin, the husband can claim that this was a mekach ta'us, that the marriage was entered into on the basis of a false presentation, and she will have no kesubah.
In summary: in this mishnah we learned the amounts of the kesubah - two hundred zuz for a virgin and a maneh for a widow; that a virgin who was widowed, divorced, or received chalitzah from betrothal alone retains the presumption of virginity and her kesubah is two hundred; and that a convert, a captive, and a maidservant who left their situation while they were less than three years and one day old have a kesubah of two hundred. And in all of these cases the husband has a claim regarding virginity, for if she is found not to be a virgin, the marriage is a mekach ta'us and she forfeits her kesubah.