Would it be considered rude or informal to call someone's father a "dad" instead of a "father"?

Native speaker’s answer
Rebecca
Not at all. Although "dad" is informal, English doesn't focus on formalities too much. It actually sounds more natural to say "your dad" or "your mom" instead of "father" or "mother." Ex: Nicholas is Courtney's dad. Ex: Her mom works with my mom. Ex: Their dad's name is George, and their mom's name is Mary.