Albert Barnes Commentary Psalms 79:8

"Remember not against us the iniquities of our forefathers: Let thy tender mercies speedily meet us; For we are brought very low." — Psalms 79:8 (ASV)

O remember not against us former iniquities — Margin: "The iniquities of those who were before us." The Hebrew can mean either former times or former generations. The allusion, however, is substantially the same.

It is not their own iniquities that are particularly referred to, but the iniquity of the nation as committed in former times. The prayer is that God would not visit them with the results of the sins of former generations, even though these were their own ancestors.

This language is derived from the idea so constantly affirmed in Scripture, and so often illustrated in fact, that the effects of sin pass from one generation to the next, involving it in calamity. See Exodus 20:5; Exodus 34:7; Leviticus 20:5; Leviticus 26:39–40; Numbers 14:18, 33; compare the notes at Romans 5:12 and following.

Let thy tender mercies speedily prevent us — literally, “Hasten; let your tender mercies anticipate us.”

The word "prevent" here, as elsewhere in the Scriptures, does not mean to hinder, as it commonly does for us, but to go before; to anticipate. See Job 3:12 (note); Psalms 17:13 (note); Psalms 21:3 (note); Isaiah 21:14 (note); Matthew 17:25 (note); 1 Thessalonians 4:15 (note).

The prayer here is that God, in His tender mercy or compassion, would anticipate their ruin—that He would interpose before matters had progressed so far as to make their destruction inevitable.

For we are brought very low — The idea in the original word is that of being pendulous, or hanging down—as vines do, or as anything does that is wilted or withered, or as the hands do when one is weak, faint, or sick.

It then refers to a failure or exhaustion of strength; the idea here is that their strength as a nation was exhausted.