It is useful to know that ‘那nà + measure word’, which appears before the noun/noun phrase after the relative clause, normally does not literally mean ‘that’, but is only used to clearly indicate the singular number of the noun.
（a）Without ‘那 + measure word’ in front of the noun, the meaning can be ambiguous.
我上星期提到的那个女孩子今天没有来参加舞会。
wǒ shàng xīng qī tí dào de nà gè nǚ hái zǐ jīn tiān méi yǒu lái cān jiā wǔ huì 。
The girl I mentioned last week didn’t come to the dance party today.
(Without 那个, it would not be clear how many girls the speaker mentioned last week.)
我昨天买的书在哪里？
wǒ zuó tiān mǎi de shū zài nǎ lǐ ？
Where is the book I bought yesterday? or Where are the books I bought yesterday?
（b）When 这zhè/那nà appears before the relative clause, it usually literally means ‘this/that’.
那个穿绿裙子的女孩就是我昨天提到的那个留学生。
nà gè chuān lǜ qún zǐ de nǚ hái jiù shì wǒ zuó tiān tí dào de nà gè liú xué shēng 。
That girl (over there) who’s wearing a green skirt is the exchange student I mentioned yesterday.
(The speaker most likely is pointing at the girl while speaking.)
这件让大家都不愉快的事，我们忘了吧！
zhè jiàn ràng dà jiā dōu bú yú kuài de shì ，wǒ men wàng le ba ！
Let’s forget this matter, which has upset everybody.
(Improper: 让大家都不愉快的这件事，我们忘了吧！)
[1] Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.
