The text below is an excerpt from the New York Times: http://www.nytimes.com/2014/06/30/world/europe/netherlands-water-management-system-global-climate-change-sea-level-rise-dutch-gene.html?_r=0
Along a rugged, wide North Sea beach here on a recent day, children formed teams of eight to 10, taking their places beside mounds of sand carefully cordoned by candy-cane striped tape. They had one hour for their sand castle competition. Some built fishlike structures, complete with scales. Others spent their time on elaborate ditch and dike labyrinths. Each castle was adorned on top with a white flag.
Then they watched the sea invade and devour their work, seeing whose castle could withstand the tide longest. The last standing flag won.
Theirs was no ordinary day at the beach, but a newly minted, state-sanctioned competition for schoolchildren to raise awareness of the dangers of rising sea levels in a country of precarious geography that has provided lessons for the world about water management, but that fears that its next generation will grow complacent.
I didn't know a possessive pronoun can be used as a subject in a sentence, so I'm not able to figure out what the word "theirs" means. Is it "their day" or "their competition"? If a possesive pronoun is the subject of a sentence, does it have special meanings?
Does the first "but" in the 3rd paragraph refer to "but rather"?
What is the grammar structure of the last clause - but that fears that? It sounds rather peculiar. Which sort of clause is it?
Could someone please tell me? Thanks a lot.