He was/had been my Professor five years ago

[joham]

The book IMPROVE YOUR ENGLISH (published in India) says we should say 'He had been my Professor five years ago' rather than 'He was my Professor five years ago'. The sentence is without a context. I feel puzzled about it. Could you explain why 'was' couldn't be used?

Thank you in advance.

 

[Barb_D]

If you are reproducing the sentence accurately, throw the book in the nearest recyling bin.

He was my professor five years ago.
He has been my professor for five years.
(They mean different things, of course.)

But not: He has been my professor five years ago.

By the way, don't capitalize "professor."