Hermione gasped. Harry and Ron wheeled round.
“Good afternoon,” he said smoothly.
stared at him.
“You shouldn’t be inside on a day like this,” he
said, with an odd, twisted smile.
“We were ––” Harry began,
without any idea what he was going to say.
“You want to be more
careful,” said Snape. “Hanging around like this, people will think
you’re up to something. And Gryffindor really can’t afford to lose any
more points, can it?”
Harry flushed. They turned to go outside, but Snape called them back.
“Be warned, Potter –– any more nighttime wanderings and I will personally make sure you are expelled. Good day to you.”
Potter and the Sorcerer’s Stone)
Why is the be verb is plain form?
The plain form, or unmarked infinitive, employed without a subject, acts in English as an imperative – a command or request.
Prof. Snape here employs the imperative passive to give Harry a formal warning; the passive voice makes Harry the “subject” of the warning and thus implies that Harry, not Snape, is responsible for any violation.
You see this very often in legal or regulatory notices:
Please be advised that your hearing is scheduled for August 15 at 10:00 pm.