Who are "ha'Am ha'Holechim ba'Choshech"?
Rashi: People of Yerushalayim, who were gloomy from worry about Sancheriv, like Chizkiyah said "Yom Tzarah v'Sochachah u'Ne'atzah ha'Yom ha'Zeh" (37:3).
Malbim: It is Shevet Yehudah. After telling how Shevet 1 Efrayim, which now succeeds, will be in darkness, Yehudah, who is now in darkness, will be light in Chizkiyah's 15th year, after Ashur falls. "Ha'Holechim ba'Choshech" implies that there is light outside, just he hides from it in an inner room. He needs only to go outside.
Presumably, he refers to Malchus Efrayim. (PF)
What was the great light?
Rashi: The fall of Sancheriv.
What does the end of the verse add?
Radak: It repeats the beginning in different words.
Malbim: "Ha'Holechim ba'Choshech" implies that there is light outside, just he hides from it in an inner room. He needs only to go outside. "Yoshevei b'Eretz Tzalmaves" is when there is no light around, e.g. at night - then "Ohr Nagah Aleihem" (like shines on them). The Nimshal is (a) Shevet Yehudah. Hashem's light and salvation shines on their land, just they hide from it and do not see the light. Then they will recognize the salvation when it comes. (b) Those (the 10 tribes) who were exiled to Chalach and Chavor (Melachim II, 17:6). Salvation is far from them (there is no light around). Then, they will return to Eretz Yisrael and take refuge in the shade of Chizkiyah.
