Hi,
Which of the following is correct? and why?
"I am sure you never had heard this before."
or
"I am sure you had never heard this before."
You can say either, but #2 is much more common. It's idiomatic, ie we like to say it that way.
Also, consider what happens when you repllace 'never' with 'not'. Now, #1 is definitely wrong. Between the auxiliary verb and the main verb is the favoured place for such adverbs.
Are you sure your context, if you have one, justifies the use of the Past Perfect?
Best wishes, Clive