才
cái
Using "cai" for small numbers
才 before a quantity means "only" — and implies it's disappointingly few.
才 + Number + Measure Word + Noun
How it works
这个班才两个学生 — this class has only two students, which is fewer than it should be.
只 also means only, but neutrally. 才 adds the judgement that the number falls short.
The same 才 marks lateness with time words; both uses share the sense of falling short of expectation.
Examples
zhè ge bān cái liǎng gè xué shēng
这个班才两个学生。
This class has only two students.
tā cái shí bā suì
他才十八岁。
He's only eighteen.
wǒ cái shuì le sān gè xiǎo shí
我才睡了三个小时。
I only got three hours' sleep.