Student or Learner
if someone ask you "what's on your desk?" and there are 2 answers for your choice.
A. there is an English book
B.it‘s an English book.
which one is correct. it's "A"or "B"?
Actually, I'd just say "An English book."
If the person can see your desk and can see something on it, but can't tell what it is, then use the form with "it." More natural for me would be "That's an English book."
If the person cannot see your desk, then use the form with "there." Or just say "An English book."
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.