When will those who trust in You rejoice?
Radak: When You will cast off the enemies who do not trust in You. Malbim - this is like "Anochi Esmach ba'Shem; Yitamu Chata'im Min ha'Aretz" (104:34-35).
Why does it say "l'Olam Yeranenu"?
Malbim: It will be unlike now, that they sing only after the affliction and salvation from it. Rather, they will sing constantly, without interruptions of afflictions.
What is the meaning of "v'Sasech Aleimo"?
Rashi: You will protect and cover them (Radak - like a Sukah, so the enemies cannot harm them).
Why will they exult in You?
Rashi: It is when they will see that You bless a Tzadik (verse 13) - Yakov and his seed (some texts - You bless Your friends and lovers).
Radak (13): It is when they will see that You bless a Tzadik (verse 13) and cast off the Rasha (verse 11).
Malbim: Their Simchah will not be natural. Rather, it is "Becha" - through realizing that You have a wondrous Hashgachah over them.
