Why does it say "Ki Ani?"?
Radak: My words are unlike those of the false seers. What I speak via my Nevi'im, it will be. This decree that I spoke to you will not be drawn out longer.
Malbim: Nevu'ah is only from Hashem, via personal Hashgachah not dependent on time, nature and causes. Astrologers tell what is destined to happen based on how the stars will be at a certain time. Perhaps the seer does not know other matters and causes tied with it that can change it, or he did not calculate the time properly. A Navi tells when Hashem will decree to bring evil. His decree brings evil to existence. His speech to the Navi is the action that brings it. It does not depend on any cause that can change. Hashem's will does not change!
What do we learn from "Adaber Davar"?
Radak: Any matter that I say, it will be done 1 .
Malbim: Via My Dibur, Ye'aseh (it will be done).
This is only for good matters! A Nevu'ah for evil can be changed via Teshuvah! Perhaps even evil decrees are fulfilled in some sense, e.g. v'Ninveh Nehepaches" (Yonah 2:21) ? it was transformed from a city of sinners to a city of Ba'alei Teshuvah! (PF)
What is "Lo Simashech"?
Rashi: It will not be drawn out 1 . Malbim ? sometimes Hashem delays a decree and His anger, for perhaps they will repent. However, in your days, Beis Meri, there is no hope that you will repent. The measure [of sin] is full. "Adaber Davar va'Asisiv" ? the Dibur itself is the action.
Radak: It is feminine, for it refers to the Gezerah.
Why does it say "Ye'aseh", and then "va'Asisiv"?
Radak: Do not say that it will happen by chance. I will do it, like I said!
Malbim: Refer to 12:25:3:1.
