Why will Hashem not forgive him?
Ramban and Seforno: Refer to 29:18:3:2. Because, irrespective of what he thinks in his heart, he entered into the covenant together with everybody else 1 (and cannot opt out).
Hadar Zekenim and Rosh (in Pasuk 18): Because, having undertaken to serve Hashem, he should not have such thoughts.
Refer to 29:18:1:3*.
Due to the principle 'Devarim she'be'Lev Einam Devarim' (Someone who enters into an agreement, any conditions that he has in mind that he does verbalize have no validity!').
What are the connotations of "Ye'eshan Af Hashem"?
Rashi: When a person gets angry, (fumes) he emits smoke from his nostrils 1 - even though this is not applicable to Hashem, the Pasuk speaks to us using language to which we can relate.
Rashbam: Hashem will not be willing to forgive, but 'His Nose will smoke'. 2
What are the connotations of "Kin'ah" regarding Hakadosh-Baruch-Hu?
Rashi: It means that He clothes Himself with vengeance and does not pardon the sinner. 1
Why does the Torah use so many expressions - "Ye'eshan Af Hashem ... ", "u'Machah Hashem es Shmo" and "Vehivdilo Hashem le'Ra'ah"?
Ramban: "Ye'eshan Af Hashem ... " refers to an individual, "u'Machah Hashem es Shmo" to a family, "Vehivdilo Hashem le'Ra'ah" to a Shevet. 1
Rosh and Hadar Zekenim: "U'Machah Hashem es Shmo mi'Tachas ha'Shamayim" teaches that someone who serves idolatry will have no remnant.
Oznayim la'Torah: "u'Machah Hashem es Sh'mo" refers to a Ger or to an individual Yisrael (or to a family), and "Vehivdilo Hashem le'Ra'ah" to a tribe, which will never be eliminated. 2
Seeing as he did not request pardon, why does the Pasuk need to say that Hashem will not forgive him?!
Lev Eliyahu (Sh'mos p.59, 61): He understands the curse, and with his mouth he requests pardon. 1
Lev Eliyahu (Ibid.): As the Pasuk writes in Yeshayah 29:13 "be'Fiv u'vi'Sefasav Kibduni ve'Libo Richak Mimeni". Hashem will not agree to forgive him - He will not help him to do Teshuvah; However, if he repents himself with a full heart, his Teshuvah will be accepted.
