I suspect that 太 has a similar function in negatives as 很 does in negative sentences.
Some Chinese speakers have corrected some of my negative sentences in include a 太.
e.g.: 他不高兴 was corrected to 他不太高兴.
How necessary is this use of 太?
What is it's function?
太 -> too, extremely, very
他不高兴 -> He's not happy -> He's unhappy
他不太高兴 -> He's not very happy -> He's a little unhappy
Just look at any Chinese grammar, e.g. Chinese, A Comprehensive Grammar, 4.3: degree adverbs and complements occur respectively before and after adjectives to indicate the degree or extent to which the meaning is encoded by an adjective is to be ascertained: 这本字典很好。[很degree adverb] 这本字典好得很[得很degree complement]. The presence of these degree adverbs and complements removes any implication of contrast that is latent in an unmarked predicative adjective. If somebody says: 这本字典好, the speaker must be implying that some other dictionary is not as good as this one.... Other degree adverbs are: 太、更／更加、万分、。。。。 Suspicion regarding function of 太 also is well known fact of Chinese grammar. Maybe it should be added that 不太高兴 (unhappy) is more common than 太不高兴 which latter expression seems to mean "not (very) happy"
