Star Wars' first lightsaber duel, in 1977's A New Hope, is a truly iconic moment that pits Darth Vader against his former master, Obi-Wan Kenobi. While certainly not as noteworthy as the face-off itself, Kenobi does something that, in retrospect, is downright bizarre: Obi Wan calls Vader "Darth," as if that were Vader's first name. Of course, at the time of the film's release, that didn't stand out as unusual. But, considering what fans know about his long history with Anakin Skywalker, it doesn't make sense at all.
Source：https://www.cbr.com/why-obi-wan-calls-vader-darth-theory/
