Be not to know
Definition 1 of 2
Pro Tip 1/3Ignorance Innocence
Indicates innocence due to a lack of information. 
He broke the vase but I'm not to know, as I wasn't there.
Pro Tip 2/3Avoiding Blame
Used to express that you can't be blamed for something you were unaware of. 
I am not to know the rules if they were never explained to me.
Pro Tip 3/3Not Expected
Stresses that it was not reasonable to expect the person to have the information. 
As an outsider, I am not to know the family's traditions.