Rashi: It is pressent tense. R. Yonah - it is present tense feminine. Present tense masculine is Nihyeh, and past tense feminine is Nihyasah. Sha'arei Teshuvah 1:31 - its form is like Nishberah. Nihyah is like "Nihyeisi v'Necheleisi" (Daniel 8:27).

Rashi #1: When man obtains his desire, it is sweet to his Nefesh.

Rashi #2: Hashem desires that Yisrael do His will. When this is fulfilled, it is sweet to Him.

R. Yonah: Desire is sweet to the Nefesh that desires only in the present, but not after it passed. Even so, they despise Sur me'Ra when opportunity for sin comes to their hands, due to the moment of pleasure. Their desire overpowers them, and they are drawn to the moment of pleasure. They are not concerned for the damage and eternal shame that will come later. See how foolish they are, and how worthless is what they get in place of [the evil that will befall them].

Sha'arei Teshuvah 1:31: When man breaks his desire also for permitted things, his Nefesh succeeds, and this Midah is sweet to it, for Sechel raised its hand and overpowered.

Rashi: A Kesil abhors veering from evil, for it is sweet to him to fulfill his desire.

R. Yonah: Sur me'Ra is when sin comes to a person, and desire overpowers him, he veers from evil.

Sha'arei Teshuvah 1:31: Kesilim who do not break their desires, and constantly pursue pleasures that people indulge in, when they will desire a sin or anything evil, they will not veer from it. They are called Kesilim for pursuing pleasures.

Malbim: A Kesil is not a Pesi. He knows the laws of Chachmah. If desire would not urge him, he would veer from evil based on the laws of Chachmah. However, since desire is sweet to the Nefesh - even though it is not good and beneficial, it is pleasant to the animal Nefesh. He cannot defeat his desire because it is pleasant. He prefers pleasant over good and beneficial. Sur me'Ra is abomination to Kesilim, for they want the pleasant.