Possible Clues:

• 'You ___ to know better'
• Something or nothing
• Demanding to have top lowered, is obliged
• 'That ___ to do it'
• 'You ___ to know better!'
• "This __ to be good"
• Is morally obliged (to)
• "You ___ to know that"
• 'There -- to be a law!'
• Expression of obligation that's repugnant in books