Why is there a Yud at the end of "Shochant"?
Radak: It is extra, like "ha'Yoshvi va'Shamayim" (Tehilim 123:1), "Chotzvi Marom Kivro" (Yeshayah 22:16) and similar verses. Also feminine [verbs sometimes have an extra Yud], e.g. "Ohavti Ladush" (Hoshe'a 10:11), "Al Tismechi Oyavti Li" (Michah 7:8). However, "Shochant" is not pronounced with the Yud, for there is a Shva under the Tav. This is like "Yoshavt ba'Levanon" (above, 22:23). It is a compound word, or Shochenes (present) and Shachanti (past). The reason is, you dwelled in the past, and you still dwell when the evil comes upon you.
Why does it say "Al Mayim Rabim"?
Radak: Bavel has much water. This is a metaphor for the great good it used to have.
Malbim: Much grain and merchandise used to come to Bavel [via its rivers].
What is in the storehouse?
Radak #1: They are of grain.
Radak #2: They are of silver and gold.
What is "Kitzech"?
Rashi: Your time [to fall].
What is "Amas Bitz'ech"?
Rashi #1: The measure of your theft.
Rashi #2: The measure of your end. This is like "Ki Yevatza" (Yeshayah 10:12).
Radak #1: The arm of your theft. You used to extort others forcefully; now the punishment comes upon you.
Radak #2: The measure that you measured and extorted Yerushalayim, so Hashem meted out to you.
Malbim: The measure that you oppressed all nations [your time has come, that so will be done to you].
