There is a rule of grammar I learned long ago and practiced for years. Now I am doubting its existence because I can’t locate it anywhere. It has to do with comparative adjectives, using the word “as.”
For example: “She is as nice as her sister.”
I was taught—or so I thought—that, when the sentence is stated in the negative, “as” becomes “so.” For example, “She is not so tall as her brother.”
It seems either I dreamt this rule or it has disappeared from modern language. I am away from home, so I don’t have my arsenal of style guides and manuals handy. A cursory search of online sources appears to prove me wrong.
Can anyone verify whether “so” was ever correct when used in the negative? I heard a host of 60 Minutes use “so” comparatively just last Sunday. The interview was on in the background, but my ears perked up because I had been contemplating this issue lately. I’d be grateful if someone could set me straight or at least bring me up to date.
Contemplating the comparative does stir two peeves within me.
The first is when a speaker follows the comparative with an objective, rather than a subjective pronoun: “She is old as us,” rather than “she is as old as we.” The “are” is implied. The night before last, I was comforted to hear Tony Bennett use the subjective pronoun correctly, through the speakers of a restaurant: “I wanna be around to pick up the pieces when somebody breaks your heart, somebody twice as smart as I.”
The second has to do with comparing two or more persons or objects. It’s fundamental, but perhaps calls for a refresher. When comparing two, the suffix “er” is used. Here, if a modifying adjective precedes the comparative adjective, “more” is used. For example: “John is the taller of the two boys.” Or “John has more children.”
When three or more nouns are being compared, the superlative comes into play. In this case, “est” is the suffix. “Mary is the eldest of the three sisters.” Or “Mary has the most freckles.”
If you have two children, you don’t refer to one as your oldest or youngest.
Notice I ended with the comparative rules about which I am most certain. In the meantime, the first—“so” versus “as”—will continue to gnaw at me. Anyone?
Word Nymph 
Using “so” in the negative is an Americanism, and correct in American English. I know: when I was still working in editorial, my job was to amend its use to “as” when the manuscript is to be put into British English. Don’t fret about it; it is right. It is just as right in British English as it is in American English, but more correct in American English. Grammar books stopped putting this rule out, and I noticed this.
Your assumption rings true. “So” is British and should stay in American.
I daren’t quibble with Johnny Mercer and neither should you, although the lyric ends with another idea for a piece. “When somebody breaks your heart like you broke mine”. Should it be “as”? It wouldn’t be so strong, would it? Woops, do I mean, “as strong”?
Where “so” so annoys is its use as a filler word to begin a conversation, as if the subject were already under discussion. This dandy device is headed for trouble. It’s the next “like”.
You make a good point. Have you noticed “so” at the end of s spoken sentence? It happens when a person lets the sentence trail off instead of coming to a definitive stop. Listen for it. Also, listen for “or” at the end of a question. You won’t read it; you’ll just hear it: “Are you in town for the weekend or?” Sadly, broadcasters interviewing people on television are just as guilty as the average teeny-bopper.
You are correct (of course). I’m not so smart!
See Similarity and Identity on this site…
http://www.multimedia-english.com/contenido/ficha/8-h-comparison-of-adjectives/1103