Q: I was taught that someone is “as good as” or “not so bad as” (rather than “not as bad as”) another. Has that changed (perhaps as a result of ignorance on the part of the speakers)?
A: In my book Woe Is I, I have a chapter on grammatical myths, and you’ve brought up one of them.
The misconception is that one should always use “as … as” for positive comparisons, and “so … as” for negative ones. (Example: “She’s as tall as her sister, but not so tall as you.”)
In fact, for many centuries, it’s been correct to use “as … as” in positive comparisons (“as pretty as ever”), and to use either “as … as” or “so … as” in negative comparisons (“not as big as before,” or “not so big as all that”).
In short, “so … as” is fine in a negative comparison, but “as … as” is correct in all cases.
In Old English, both “as” and “so” (als and swa) once appeared in the same word, alswa, which was used in comparisons pretty much the way we use “as” these days (“alswa good alswa gold”).
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