Sotah, Chapter Two, Mishnah Five
After the curses are recited, the woman responds Amen, amen. Amen is a Biblical expression of agreement. Due to it frequent use in prayer, it is to this day one of the most familiar Hebrew words (my mother says that when pregnant with me I used to kick when my father said amenJ).
Our mishnah deals with the repetition of the word amen. Why does she say the same word twice? To what exactly is she responding Amen, amen? Note that the mishnah accepts a multiplicity of answers to these questions, for they do not necessarily contradict each other.
To what does she respond Amen, amen?
1) Amen over the curse and amen over the oath;
2) Amen with respect to this man and amen with respect to any other man.
3) Amen that I did not go astray as a betrothed girl or married woman or a shomeret yavam or a woman taken into [her yavams] house.
4) Amen that I have not been defiled and if I have, may [these curses] come upon me.
5) Rabbi Meir says: Amen that I have not been defiled and the amen that I will not become defiled in the future.
Section one: Verse 19 contains an oath to which she must agree. The oath is that she is not guilty of the sin of adultery. Verse 21 uses the word alah, a curse that if she has committed adultery the following things shall happen to her. According to the first interpretation of amen, amen in our mishnah, the first amen is to the curse and the second amen is to the oath.
Section two: Although the warning she received not to seclude herself, had been specific to a certain man, i.e. dont seclude yourself with so-and-so (see chapter one, mishnah one), our mishnah teaches that when she drinks the water, it tests her for adultery with others as well.
Section three: She must affirm that she did not commit adultery at any point in the marriage. In a regular marriage this means both during betrothal and after marriage. In a levirate marriage this means from the time that her husband died while she was waiting for her brother-in-law to marry her (this is the shomeret yavam, for more info see the intro to Yevamot), or after she had entered her yavams (her brother-in-laws) house as his levirate wife.
Section four: This is a different type of interpretation of the words amen, amen. The first amen is her affirmation that she did not commit adultery; the second amen is her acceptance of the curses that will be visited upon her. Some commentators say that this section is an expansion of the first section.
Section five: Rabbi Meir believes that the waters are effective even for future adultery. At this point she affirms that she has not committed adultery in the past and that should she do so in the future, the waters will bring their curse upon her at that later time.