Can anybody explain the usage of the word Khun in Thai (possibly a range of uses, I suspect), it's specific meaning and its propriety in various social contexts? Or, point me to an online discussion of this ?
It can mean like Mister: Khun John (Mister John)
It is a polite way of saying "you": khun ja pai nai (where are you going)
It can be very formal or polite meaning like Sir: Khun Tun (Respected Sir; Tun is a polite word indicating social superiority)
The whole question of personal pronouns is generally difficult in any East Asian language and in Thai they are somewhat in a state of flux; the website's information is a bit out of date, but not really wrong. The further away from Bangkok you get the more the traditional forms are observed. As the article points out, the correct word for "you" in print is ท่าน.