I read this news article, where 20-year-old Mollie Tibbetts went missing. Her name is Mollie Tibbetts. If I get it correct, Mollie is her first name and Tibbetts is her last name or surname. As we can see from the article, most of time the author uses Tibbetts, like:
Laura Calderwood, Tibbetts' mother, told ABC News there are ...
Tibbetts, who was the middle child in her family, ...
There is one time her mother uses Mollie:
Calderwood said. "As that date gets closer, it's like closing in on me, because I'm thinking about these two people that are getting married that were also really close to Mollie. [It's] supposed to be the happiest day of their life."
Also, her mother's name is Laura Calderwood, but the author uses Calderwood all the time in his article, instead of Laura.
From my observation, in our daily speech, we tend to use people's first name to address someone. For example, I address one of my American friends as Brain, which is his first name.
So, I'm wondering if there is a rule or something? In other words, in which case(s) we should address someone's last name/surname or his first name?