Jack McCaulk wrote:
This is extraordinarily basic grammatical stuff and on this basis alone you deserve to fail every language course you have taken, are now taking and will take in the future.
Yes it is basic. It's a shame you still have it wrong.
"One" belongs to a a group of people. The group of people is collectively referred to as "those who care".
Yes, "one" is the subject of the phrase. Good for you for noticing! Yet "one" is not the word referred to by "care". The fact that "those" is preceeded by a preposition is irrelevant. Being part of a "prepositional phrase" does not confer some magical ability upon it to change the basics of the language.
If the purpose of the phrase had been to distinguish the "one" from "those", then "cares" would be the proper word. As in, "he's one of those, except that unlike them, he cares". However this was the not meaning conveyed.
Class dismissed.
Keep trying.