Use of 不太 and when is it necessary? 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