Rashi: When I lowered Melech Ashur in the hand of his enemies.

Malbim: Nebuchadnetzar conquered Ninveh in his first year (Megilah 11b), and from then he began to conquer many nations. All were handed over to him, until he conquered Egypt and destroyed it; that was the end of his wars. The verse depicts that on the day that he destroyed Ashur, it was decreed that he destroy all the kingdoms that initially were under Ashur.

Rashi: I made many (who relied on him) mourn over him.

Radak: I will make another mourn (refer to 31:15:3:2); this is Po'el Yotzei (it acts on another).

Rashi: I covered the depth for him. It used to water him and raise him ? "Tehom Romemas'hu" (4). That day, I covered it, lest it extinguish his burning, i.e. I weakened his helpers.

Radak: It is as if the Tehom, which used to raise him, mourns over him when he is cut off. He was compared to a cedar, and it said "Tehom Romemas'hu" ? now, it is as if the Tehom will mourn over him and wear sackcloth. Kisesi is Po'el Yotzei Shelishi (one causes a second to act on a third party ? He made the Tehom cover itself).

Malbim: The Tehom became a mourner. Just like a mourner covers his head and does not go outside, so Hashem covered the Tehom, so it will not go out to water the trees.

Rashi: Those that go around where it is planted.

Radak: They are the rivers of the Tehom, which go around where it is planted.

Rashi: The water lengthened its branches - "va'Te'erachnah Porosav mi'Mayim Rabim" (5).

Rashi: I will blacken [Levanon for him].

Radak: Because he was compared to a cedar of Levanon, it says that when he falls, all trees of Levanon were darkened. All of this is in the first person (He'evalti, Kisesi, va'Emna, va'Akdir) to inform that the evil came to him from Hashem.

Malbim: All Levanon was blackened due to burning heat, for there was no water.

Rashi: Irrigation channels took water from [the rivers] to all trees. Their moisture ceased. This is like one who faints, and his Ru'ach flutters.

Radak #1: Ulepeh is an adjective, like "va'Yis'alef" (Yonah 4:5), "Tis'alafnah ha'Besulos" (Amos 8:13). It is great exertion and panic, to the point that one cannot hold himself back, like Chazal said Shema Yis'alfah (Shabbos 9b).

Radak #2: Ulepeh is a verb ¹ , with an extra Hei at the end; the subject is not mentioned. We find an extra Hei at the end, e.g. "Isheh la'Shem" (Vayikra 2:11), "Tzachah Tzamei" (Yehoshua 5:13). The singular is used, for each tree was anxious.

Malbim: All trees were faint amidst thirst, like a man faints amidst heat and thirst. I.e. then the influence ceased from all kings who took refuge in the shade of Ashur, for there was no one to protect them from the afflicter's anger.