Who will say so to Yerushalayim?
Radak #1: The Nevi'im will.
Radak #2: One will say so to his colleague.
Radak #3: The enemies will say so. They will admit that they do not have strength against Hashem.
Why does it say "Al Tira'i" and "Al Yirpu Yadayich"?
Malbim: The nation dwelled in Yerushalayim; it was the civil gathering. There, they feared the enemy. Tziyon was the place of the Sanhedrin and spiritual matters. It slackened from engaging in perfection of the Nefesh, for it thought that Hashem abandoned it due to its sins. It should not slacken!
Why does it say "Yirpu" (masculine)? The subject (Yadayich) is feminine!
Radak #1: We find like this - "[ha'Nashim] Yitenu Yekar" (Esther 1:20).
Radak #2: Sometimes Yad is masculine - "v'Hinei Vo Megilas Sefer" (Yechezkel 2:9).