“Thou shalt keep them That is, the persons referred to in Psalms 12:5 - the poor and the needy who were suffering from the wrongs inflicted on them. The idea is, that God would guard and defend them. They were safe in his hands. Compare Psalms 37:3-7.
From this generation - This generation, or this race of detractors, flatterers, and oppressors. The idea is, that that entire generation was eminently wicked, and that none but God could deliver the poor and the needy from their designs.
Forever - That is, “constantly,” or as long as they would need the divine protection. God would not interpose and save them from the “present” trouble, and then leave them to the designs of their enemies, but he would “always” interpose as often as there was any need of his help. That is, they were now, and would be at all times, entirely safe. They had nothing to fear, for God was their refuge and their help.