What is the question "until when"?
Rashi: Until when will they prophesize Sheker? Do these false Nevi'im intend to fulfill their intent, to make My nation forget My name (verse 27)?
Radak: Do they intend to do so forever?!
Malbim: Until when will they switch dreams with (claim that they are) prophecy?! Moreh ha'Nevuchim explains that a Navi can distinguish a simple dream, which is via imagination, and a prophetic dream from Hashem.
Why does it say "ha'Yesh b'Lev ha'Nevi'im"?
Malbim: Do they think in their hearts that it is a prophetic dream?! When a Navi had a prophetic dream, he had no doubt that it is lasting Emes. Either they fabricate Sheker, or 1 their hearts deceive them.
Hashem had no Safek! Perhaps Malbim means that (a) some fabricate Sheker, and some, their hearts deceive them, or (b) We may infer that one of these is the case. The verse did not teach which.
What is the grammatical form of "Nib'ei"?
Radak: It is a verb conjugated in the Nif'al.
What is "Tarmis"?
Rashi: It is an expression of Mirmah (deception). Radak - they prophesize what they think in their hearts, and not Nevu'ah that came to them.
