O remember not against us former iniquities: let thy tender mercies speedily prevent us: for we are brought very low.
O remember not against us former iniquities - rather, 'the iniquities of our forefathers.' Leviticus 26:45 confirms this: after threatening in Ps. 79:39 , "In the iniquities of their fathers shall they pine away," God proceeds to promise, "If they snail confess their iniquity, and the iniquity of their fathers ... I will for their sakes remember the covenant of their ancestors." The Jews now plead this promise, after they have experienced the fulfillment of the threat (cf. Lamentations 5:7). The children, when innocent, are not punished for the guilt of their fathers, but for personal guilt: the entail of guilt might be cut off by repentance and faith; but the guilty children, inheriting the sinful dispositions of their fathers, sought not the Spirit of God to create them anew, but followed their fathers' steps and exceeded them in sin, hardening themselves the more in proportion as the long-suffering of God delayed to execute judgment. Thus the accumulated guilt of fathers and sons fell in vengeance on the last generation. 'They (here) acknowledge an obstinacy of long standing, in which they have hardened themselves against God. Sacred history testifies that the punishment of the captivity was postponed until God had proved that their wickedness was incurable' (Calvin). (Isaiah 64:9.)
Let thy tender mercies speedily prevent us - i:e., come steadily and unexpectedly, taking us by a joyful surprise (Psalms 21:3, note).