Why does it say "Yivaled Lanu"?
Rashi: Even though Achaz was a Rasha, his son born several years ago to be king after him, will be a Tzadik who serves Hashem.
Radak: The plague in Machaneh Ashur will be in the days of the son (Chizkiyah); it is as if it is in his merit.
Malbim: The salvation will be due to a new occurrence and Mazal.
Why does it add "Ben Nitan Lanu"?
Radak: The matter is repeated in different words.
Malbim: The son will inherit the kingship.
Why does it say "va'Tehi ha'Mishrah Al Shichmo"?
Rashi: Hashem's yoke will be on him; he will engage in Torah and guard Mitzvos and lean his shoulder to [carry] Hashem's load.
Radak: Shichmo always refers to a burden, like "va'Yet Shichmo Lisbol" (Bereishis 49:15), "v'Es Mateh Shichmo" (above, 3), "Yasur Sivlo me'Al Shichmecha" (10:27). Why is it said here about Mishrah (authority)? It is because Achaz served Melech Ashur; he bore [his yoke] on his shoulder. The verse teaches that the child will not have service on his shoulder, rather, authority.
What will be his name, and what is its significance?
Sanhedrin 94a: He has eight names - Pele Yo'etz El Gibor Avi Ad Sar Shalom. Chizkiyah was not his true name. He is called so because Chizko Kah (Hashem strengthened him), or because Chizek (he strengthened) Yisrael to serve Hashem.
Rashi: Hashem, who is Pele Yo'etz (wondrous counsel), Kel Gibor and Avi Ad, will call him Sar Shalom, for there will be Shalom and Emes in his days.
Rashi (6): "Pele Yo'etz
