Exercise 6.5 The 'so' of
She does so know Chinese.
is a marker of positive polarity. It is obligatorily stressed when it appears and functions to indicate insistence on the truth of a sentence. Just like negation it requires periphrastic 'do' if and only if the sentence contains no tensed auxiliary. Thus, it is the positive counterpart of emphatic (stressed) negation:
One issue raised by this approach, however, is how to account for the fact that 'so/too' occurs mainly in tensed main clauses and cannot occur in infinitives.