>> Or, in US English, "it's so a common mistake".
>>
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> "It is so a common mistake" might be part of an argument. Note the
> separate words, indicating at least an equal stress on the "is so".
I have the usage of "so" that Steve is alluding to, but I can't use it in
the sentence in question - in particular, I don't think it can be used
where it can be grammatically replaced by "such" (e.g., before a noun
phrase with "a"/"an"). This is the usage of "so" to modify the complement
of the copula "be" in order to emphasize the state of affairs indicated
by the sentence. "So" is stressed; the copula usually isn't. For instance,
"You are so not my friend anymore"; "As soon as I get home, I'm so going
right to bed."
This is different from the use of "so" you describe, which has the
meaning 'I reassert my claim in the face of your denial.' This "so", as
you note, requires stress on both "so" and the verb auxiliary, and it
can't be used with "not", so this can't be the "so" in either of the
example sentences above.
> "It is a so common mistake" might also be heard.
I don't think this is grammatical for me. It'd have to be "such a common
mistake" or "so common a mistake".
-Aaron J. Dinkin
Dr. Whom