Thou shalt keep them, O Lord. God having promised to set the righteous, who are oppressed, in a place of safety (Psalms 12:5), the psalmist is sure that he will keep them and preserve them from the wicked "generation," which has possession of the earth, and bears rule in it, always.
It is, no doubt, for the greater consolation and encouragement of these unfortunates that he dwells on the subject, and adds his own assurances to the Divine promise which he has recorded. Man's faith is so weak that, unless promises and assurances are reiterated, they make little impression.
Thou shalt preserve them (Hebrew, him) from this generation for ever. The "generation" is that of the worldly men in power at the time, of whom we have heard in Psalms 3:1, Psalms 3:2, Psalms 3:6, Psalms 3:7; Psalms 4:2; Psalms 5:4-6, Psalms 5:9, Psalms 5:10; Psalms 6:8; Psalms 7:1, Psalms 7:2, Psalms 7:9, Psalms 7:13-16; Psalms 10:2-11, Psalms 10:15; Psalms 11:2, Psalms 11:3, Psalms 11:6.
"For ever" means "so long as they live." The substitution of "him' for "them" in this clause is an instance of that generalization by which a whole class is summed up in a single individual—" all men" in "man," "all good men" in "the righteous" ( צַדִּיק), and the like.