I dunno - there after plenty of cases we use "the" despite it not referring to a known single instance of something. "The thing is..." or "what's the matter" or

I dunno - there after plenty of cases we use "the" despite it not referring to a known single instance of something. "The thing is..." or "what's the matter" or even "I've got the flu". Good luck trying to explain to Japanese speakers especially why "a dinner" or "go to a hospital" don't sound natural but "I'll have a beer" or "a few friends" are fine. And why is it "once upon a time" but "most of the time"...