Complex sentence: Puyi could not speak Manchu, he only knew a single word in the language, "Yili" (which meant arise), despite studying Manchu for years, he admitted that it was what he was "worst" at it among everything he studied.
Split into simpler sentences: Puyi could not speak Manchu; he only knew a single word in the language, "Yili," which meant arise. Despite studying Manchu for years, he admitted that it was his "worst" subject among everything he studied.