Búdà不大 and bùtài不太 both mean ‘not very…’ and are frequently used in the structure of ‘不大/不太+Adjective’. When adjectives express the positive meanings, they are interchangeable, for example,
1)他不大高兴
Tā búdà gāoxìng
He not very happy
‘He is not very happy.’
2)他不太高兴
Tā bùtài gāoxìng
He not very happy
‘He is not very happy.’
From the example, we can see that the adjective is gāoxìng 高兴 happy, which express the positive meaning. However, when the adjective expresses the negative meaning, it can frequently be matched with bùtài不太, but is seldomly allowed to match with búdà不大, for example,
3) 不太笨
Bùtài bèn
‘Not too stupid’
4)*不大 笨
Búdà bèn
