What are the implications of the phrase "Asher Lo Tzivisiv Le'daber"?
Rashi: It is what Hashem did not command him to say, though He did command another Navi to say it, 1 only he said it in his own name. 2
Seforno (in Pasuk 21): The Pasuk is speaking about a Navi who comes to institute a temporary 'Mitzvah'. 3
Why does the Torah insert the (otherwise superfluous) word "asher Yazid Le'daber Davar"?
Sanhedrin, 16a: To teach us via a Gezeirah Shavah "Davar" "Davar" from Zaken Mamrei - 17:11, 1 that the Navi Sheker is judged and sentenced to death by the Sanhedrin of seventy-one.
See Torah Temimah, note 74.
What sort of prophecy is the Torah referring to when it writes "va'asher Yedaber be'Shem Elohim Acherim"?
Rashi and Ramban (both citing the Sifri): He is guilty, irrespective of the contents of his prophesy, even if it conforms to the Halachah - for example, if he announces that Pe'or, who is god, 1 appeared to him and informed him that it is a Mitzvah not to eat Chazir, or he predicts that such and such is going to happen. 2
Which of the four death-penalties does the Navi Sheker receive?
Targum Yonasan: Refer to 13:6:25:2 and note.
The lightest of the four Misos.
Rashi: In fact, three of the current 'prophets' are Chayav Misah at the hand of Beis-Din - One who prophesies 1. ... what he did not hear; 2. ... what was not said to him but to somebody else; 3. ... in the name of Avodah-Zarah. See Torah Temimah, citing Sanhedri, 89a #2.