Student or Learner
1) I will be lying on the beach at 5 pm.
Question: Is there only one possible meaning of the action starting before 5 pm?
2) I will be lying on the beach in 5 hours.
Question: What about this example? Will the action of lying start in 5 hours or does
it mean that the action will start before and already be in progress in 5 hours?
Greetings from Bavaria
It will be in progress by then. You don't know if it will start at 4:59 pm and so that by 5 pm it is in progress, or if it will start a minute from now.
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.