When using if and can't they both have the similar "self doubt" aspect in them. Like "If I can do it..." and "I can"t do it." Both of these statements are basically questioning their self worth.
Because proper Table manners are respectful especially in a public place. And its The proper thing to do. someone’s that being respectful, makes things less awkward And more comfortable
The Stamp Act pretty much made all American Colonists pay tax on anything printed.