18 verses. King Solomon opens his book declaring that everything is cyclical, and not moving forward, that oblivion is the fate of all, and that search after the ultimate wisdom is a recipe for pain and frustration. He cites his credentials as the wisest of all men until his time, as an indefatigable researchers into matters of truth and reason, concluding implicitly that his realization of futility of reality, its doomed oblivion, and its ultimate cyclical nature are beyond challenge.
26 verses. Kohelet tries to escape from the conclusion of meaninglessness by building real estate and enjoying the good life, but it is all in vain, nothing makes sense. Wisdom to stupidity is like light towards darkness, but the end of the wise and the stupid is the same, so wisdom is no edge.
12 short verses. 8 of them are poetic phrases of contradiction: life, death; planting, tearing; killing and healing; breakup and construction; crying and laughing; obituaries and dances; throwing stones, gathering stones; hugging, refraining from hugs; to seek, to abandon; to keep, to throw away; to tear, to sew; to talk, to remain silent; to love, to hate; to make war, to make peace.
And then (9): what is the man's advantage, cause -- his labor, his work! Following Kohelet shares from the heights of his wisdom: "I have seen the matter God gave people to ponder". God made everything to have beauty, if as the poetry above says, it is all fit in good time. And God gave the knowledge of the universe into man's heart, but it is encrypted, so to prevent humans from comprehending the divine plan in its entirety.