1)
In what will they rejoice?
1.
Radak #1: One who trusts in Hashem will rejoice from the great good that comes to him from Him.
2.
Radak #2: His Nefesh will find great Simchah in Avodas Hashem, and in his trust in Him.
2)
What is the meaning of "Harninu"?
1.
Radak: It is Po'el Yotzei (it acts on others). Everyone whose heart is straight and he wants to adopt your path, make him rejoice and sing.
2.
Malbim: Those whose hearts are straight should sing, for all He does is for your benefit and success.
