“the dead do not know anything.” When a person dies they are dead, lifeless, in every way. They are not alive in heaven or “Hell” or any other place, they are dead, as Ecclesiastes 9:5-6 describes. Because dead people are actually dead, they “do not know anything.” The grave is called “the land of forgetfulness” (Psalm 88:12) and it is a place of silence (Ps. 94:17)
[For more on people being dead and in Sheol, the state of being dead, see commentary on Rev. 20:13. For more on dead people being lifeless in every way, see Appendix 4, “The Dead are Dead.” For more on what the “soul” is, and that it does not live on after a person dies, see Appendix 7, “Usages of ‘Soul’”].
“nor do they have any more a wage.” The Hebrew word translated “wage” is sakar (#07939 שָׂכָר), and its primary meaning is “wage, hire, fee, pay,” and it is also sometimes used with the more expansive sense of “reward, benefit.” Although most English Bibles read “reward” or “benefit” or something similar, there is no reason not to read “wage” here. In fact, “wage” makes excellent sense in the scope of Scripture.
People earn “wages” in the spiritual world while they are living. Thus, Proverbs 10:16 says, “The wage of the righteous person is life; the revenue of the wicked is sin” (Prov. 10:16 uses different but applicable Hebrew words for “wage.” Cp. Prov. 11:18). In the context of evangelizing, Jesus said that the person who reaps receives wages (John 4:36). Romans 6:23 says the wages of sin is death, and 2 Peter 2:13, speaking of unsaved people who will be destroyed, says they will suffer wrong as the “wage of wrongdoing.”
Once a person dies they do not earn any more wages—they have what they have earned. The sinner has earned death, and the righteous person has earned everlasting life. However, there are some people who teach that there will be a “second chance” on Judgment Day and that people who are raised from the dead will have a chance to confess their sin and be saved. That is not the case. Scripture says that we have this life to live and please God, and after this life comes the judgment (Heb. 9:27).
So in the context of the scope of Scripture we can see why the Bible would say that dead people “do not have any more a wage,” because they cannot earn a wage anymore—they are dead. So Ecclesiastes 9:5 is an encouragement to righteous people to keep working to earn “wages,” i.e., rewards in the future. It is also an encouragement for sinners to get saved and earn the “wage” of everlasting life, because if they die before getting saved they will not have another chance—they will be resurrected and awake to Judgment Day and their doom.
“the memory of them is forgotten.” That is, the person is forgotten. In the Jewish world, that people are remembered after death is very important. That people will eventually be forgotten is stated elsewhere in Ecclesiastes (cp. Ecc. 1:11).