As a British native English speaker I have only ever used the word "folk" as an adjective in phrases such as "folk music" or "folk tales". My unscientific impression is that its use as a synonym for people is increasing in the British media, which prompts the question: How can I decide whether it should be folk or folks?
- We should do more for our old folk.
- We should do more for our old folks.
- Some folk never listen.
- Some folks never listen.
Are the two words interchangeable, or are there some clear usage patterns?