"I am under orders not to let anyone in"
In here, why "s" is added to "order"? Only one order, "not to let anyone in" is given to the subject, isn't it? Please help me again.
It's simply idiomatic to say "orders."
Your orders are simple: Don't let anyone in!
I'm just following orders.
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.