Nàge dàxuéshēng, jīnnián cái/*jiù shíwǔ suì.
(That college student is only fifteen years old this year.)
Yī xuéqī *jiù/cái niàn le yī běn shū, hái méi niàn wán.
(We have read only one book in a semester, but we still haven’t finished it.)
Zuótiān kāihuìjiù/*cái xiǎo wáng yī rén méi cānjiā.
(Only Xiao Wang didn’t attend the meeting yesterday.)
The expectation in (1a) is that a college student is usually eighteen years old or older, while that student’s age is only fifteen. Thus, the contrast between that student’s age and the average age of college students is a scale of contrast, and ‘cái’, rather than ‘jiù’ indicates such a scalar contrast (Chu, 1998:100). On the other hand, the expectation in (1c) is that everybody should attend the meeting. So the contrast is between the people attending the meeting and the people absent from the meeting—which is a simple contrast. Thus ‘jiù’, instead of ‘cái’, is used for such a contrast.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.