Does Psalms 37:28 teach perseverance of the saints and once saved, always saved? (Psalms 37:28 For the Lord loveth judgment, and forsaketh not his saints; they are preserved for ever: but the seed of the wicked shall be cut off.) The preserved forever part.
"I have been young and now I am old, yet I have not seen the righteous forsaken or his descendants begging bread. All day long he is gracious and lends, and his descendants are a blessing. Depart from evil and do good, so you will abide forever. For the LORD loves justice and does not forsake His godly ones; they are preserved forever, but the descendants of the wicked will be cut off. The righteous will inherit the land and dwell in it forever" (Psalms 37:25-29).
Like most questions that are based on a single verse, you usually find your answer by reading the verses around it. David is discussing the righteous and their descendants versus the wicked and their descendants. David is stating that the wicked tend to die young so they won't remain in the earth for long. However, because God cares for the righteous, they and their descendants tend to remain. Jesus said much the same thing, "Blessed are the gentle, for they shall inherit the earth" (Matthew 5:5).
The Jewish concept of "forever" was an indefinite, long period of time. While the Israelites remained faithful to God, God would see to it that they remained in the land of Israel. But if they turned wicked, they would be removed.
This doesn't apply to individuals, but to family lines. It doesn't state that people once declared righteous would remain righteous nor does it say that the wicked have to remain wicked. In fact, it specifically states that the wicked can become righteous and partake of the blessings. But implied in this is if the righteous become wicked that they would lose the blessings.