DEAR MISS MANNERS: Degradation of the English language has become a prickly burr under my tail. While speaking with someone, if they say 6 times 5 equals 35, it is not considered impolite to correct them and say no, it’s 30. So why is it considered impolite to correct someone’s grammar?
GENTLE READER: Even in the halcyon days when we naively thought that facts were immutable and universal, there were problems with your premise. Miss Manners has no trouble imagining rude ways to correct a person’s multiplication.
As she is in the business of making the world more polite, not less, she will limit herself to one general example: delivering said correction in a way that belittles or embarrasses the recipient. It is to avoid such a result that etiquette has rules against correcting others. And of course, there are exceptions -- responding to a request, teaching, saving someone from a worse fate (like underpaying the dinner bill) -- many of which still require careful handling.