What is the grammatical form of "Yisad"?
Rashi: It is past tense, like "Yisad ha'Melech" (Esther 1:8). I already decreed that I will set up Melech ha'Mashi'ach to be in Tziyon Even Bochan.
Radak: It is an adjective.
What is "[Even] Bochan"?
Rashi: It is an expression of a fortress and strength, like "Ofel va'Vachan" (32:14), "Hekimu Vechunav" (23:13).
Radak: I will establish a rock in Tziyon - Melech ha'Mashi'ach. He is compared to a big rock used for a foundation. Big rocks are used also for the cornerstones, to sustain the building. A good king will reign in Tziyon; via him, it will be saved from the enemy. This is Chizkiyah. He will make the nation repent, and eradicate Resha'im.
Malbim: The verse depicts that Beis Yehudah was destroyed, and Hashem begins to build it afresh. The Machseh Chazav will depart (17), and Chizkiyah will be the new foundation. It has three attributes. (a) The rock is strong due to its physicality. It is a tested rock; it is not composed of earth, sand and foreign matters. (b) Its value and shape make it proper for a cornerstone. It is precious, like sapphire or carbuncle. (c) Its purpose - it is the foundation of foundations 1 .
Malbim: The Nimshal is, this king will be tested to be a total Tzadik clean of desires and his thoughts are clear; his lineage is from Beis David; and he will be the foundation of foundations for counsel and Gevurah, and he will begin to save Yisrael.
Why does it say "Musad Musad"?
Rashi: The first is Samuch - a foundation of a foundation, an absolute foundation.
Radak: The first is a noun; the second is Pa'ul (it was founded). This will be the strongest foundation, i.e. Chizkiyah.
What is "Yikras "?
Radak: This is a noun, like Yekar. The Sov at the end is in place of a Hei. It is not Samuch, like I explained about Tzitzas (refer to 28:4:1:1).
What is the meaning of "ha'Ma'amin Lo Yachish"?
Rashi: One who believes this will not rush it. He will not say 'if it is true, let it come quickly 1 ' (like others said - refer to 28:10:3:1). Radak - most of them did not believe the Navi's words. The believers will see it - if it will not be so close, it will not be far.
Malbim: For first there will be great afflictions.
