Student or Learner
Has somebody stolen my pen?
Has anybody stolen my pen?
--- Which is better?
I don't see much difference, but the "anybody" form seems more likely to mean "any of you people listening" versus "somebody, who may be here or may be anywhere."
(Off topic: I don't know too many people who would admit to having stolen a pen, though. Try "borrowed" and you're more likely to get it back.)
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.