A generation goes, and a generation comes, but the earth remains forever.
To prove the vanity of all things under the sun, and their insufficiency to make us happy, Solomon here shows, 1. That the time of our enjoyment of these things is very short, and only while we accomplish as a hireling his day. We continue in the world but for one generation, which is continually passing away to make room for another, and we are passing with it.
Commenting on Ecclesiastes 1:4-8
One generation passeth away, and another generation cometh,.... This shows that a man can have no profit of all his labour under the sun, because of his short continuance; as soon almost as he has got anything by his labour, he must leave it: not only particular persons, but families, nations, and kingdoms; even all the inhabitants of the world, that are contemporaries, live together...
earth . . . for ever-- (Psa 104:5). While the earth remains the same, the generations of men are ever changing; what lasting profit, then, can there be from the toils of one whose sojourn on earth, as an individual, is so brief? The "for ever" is comparative, not absolute (Psa 102:26).