Why does the Torah juxtapose "mi'Kol ha'Ochel" next to "Kol asher be'Socho Yitma"?
Rashi: To teach us that it is only food that can receive Tum'ah from a K'li Cheres but not vessels. 1
Because the Tamei Keili is a Rishon le'Tum'ah and Keilim can only receive Tum'ah from an Av ha'Tum'ah (Rashi). By the same token, we can extrapolate that if a Sheretz falls into the oven, the oven becomes a Rishon (and any food that is inside it, a Sheini) - and not an Av, like the Sheretz (as is the case by Tum'as Meis); because if it did, there would be no reason to preclude Keilim from adopting the same level of Tum'ah as the Sheretz (Rashi).
What are the connotations of the words "asher Ye'achel"?
Rashi: It implies that the Shi'ur Tum'ah regarding food is a k'Beitzah - since the maximum that the throat can swallow in one go is an egg.
What is the significance of the words "asher Yavo alav Mayim Yitma"?
Rashi: This teaches us that food only becomes subject to Tum'ah (b'chi Yutan) once it has had contact with water. 1
And the status remains even after the water has dried (See Rashi on Pasuk 38, DH 've'Nafal').
What is the reason for this distinction?
Ramban (on Pasuk 37): Because when the food is wet, the filth of the Sheretz or of whatever is transmitting the Tum'ah, sticks to the food. 1
Rashbam: Because Hashem did not require food to become subject to Tum'ah until one has begun preparing it to eat; and the first major preparation of food is that of adding water.
And the Torah extended the Chumra even after the water has dried off, in order not to differentiate between one situation and another ('she'Lo Nitein Devareinu le'Shi'urin' [Ramban]). Acording to Rashi (in Pasuk 38) it is because, once the food becomes b'chi Yutan, the status cannot be removed.
Why does the Torah add "O al Kol Mashkeh asher Yishaseh"?