Sotah Chapter 2, Mishnah 5. This Mishnah addresses the wording of the Torah, which states that after the woman is placed under oath she says "amen, amen". From the repetition of the word "amen" we derive the concept of gilgul shevuah, the extension of an oath: once a person is already obligated in an oath, additional matters may be attached to it and he may be placed under oath about them at that same time. This principle applies to the sotah as well, as we will see in our Mishnah.
The Mishnah asks: "Al mah hi omeres amen, amen?" - regarding what does she say "amen, amen"?
In other words, which additional matters are included in this statement:
"Amen al ha'alah, amen al hashevuah" - amen to the curse that is included in it, and amen to the oath she was made to swear.
"Amen me'ish zeh" - that she was not defiled with the man about whom her husband warned her not to seclude herself.
"Amen me'ish acher" - once she is swearing, the husband can extend the oath to cover possible relations with another man as well.
"Amen shelo satisi arusah unesuah" - not during the period of betrothal, before they began living together as husband and wife, and not after the marriage either.
"Shomeres yavam ukenusah" - an oath relating to a woman who came before a yavam for levirate marriage.
The oath of a shomeres yavam:
The case is a woman who underwent levirate marriage and is now married to the yavam. If the yavam expressed jealousy, warned her, and she then secluded herself with that man, when she is given the sotah waters to drink she can be placed under oath not only regarding the period of her marriage to the yavam ("kenusah"), but also regarding the period when she was a "shomeres yavam": after her husband's death, when she was forbidden to marry anyone other than the yavam. If the yavam wishes, she also swears that she had no forbidden relations during that time.
It is worth noting that this ruling follows the view of Rabbi Akiva, who holds that a shomeres yavam who had relations with a man other than the yavam becomes forbidden to him, and she then requires only chalitzah. But the halachah does not follow Rabbi Akiva: although a shomeres yavam is forbidden to have relations with anyone other than the yavam, if she did so she does not become forbidden to him, and therefore he cannot place her under oath about it.
The Mishnah continues: "Amen shelo nitmeisi" - that I was not defiled through relations; "ve'im nitmeisi" - and if I was defiled, may the waters come and do their work in me.
The dispute between Rabbi Meir and the first Tanna:
Rabbi Meir says that the statement "amen shelo nitmeisi" includes an oath regarding the future as well - that I will not become defiled from here on - and in his view the sotah waters can affect her in the future too. The first Tanna disagrees and holds that there is no concept of an oath about the future that would have any effect.
In summary: In this Mishnah we learned the law of extending an oath, derived from the repetition of "amen, amen": the woman swears regarding the curse and regarding the oath, regarding this man and regarding another man, regarding the period of betrothal and of marriage, and even - according to Rabbi Akiva - regarding the period of being a shomeres yavam and of the levirate marriage itself, as well as "that I was not defiled" and that if she was defiled the waters should do their work in her. And the first Tanna and Rabbi Meir dispute whether this oath also includes a commitment regarding the future.