1)
Why does it say "va'Ya'amod"?
1.
Malbim (Yeshayah 36:13): He addressed the entire nation. He spoke loudly, in Yehudis.
2)
Why did he say "ha'Melech ha'Gadol"?
1.
Malbim (Yeshayah 36:13): This implies that Chizkiyah is a small, weak king.
