When the relative clause is ‘subject + verb’ and the noun which the relative clause modifies is the object of the verb, an optional 所 can be used before the verb in a more formal style of speech.
老师(所)说的话，你们都懂了吗？
Lǎo shī (suǒ )shuō de huà ，nǐ men dōu dǒng le ma ？
Did you all understand the words (which) the teacher said?
(话 is the object of the verb 说; therefore, an optional 所 can appear before 说.)
人口太多(所)带来的问题，是很不容易解决的。
Rén kǒu tài duō (suǒ )dài lái de wèn tí ，shì hěn bú róng yì jiě jué de 。
The problem (which) excessive population has brought is very diffi cult to solve.
(问题 is the object of the verb 带来; therefore, an optional 所 can appear before 带来.)
When the relative clause is ‘subject + verb + indirect object’, an optional 所 can be used as well.
我(所)告诉你的那个秘密，请你不要告诉别人。
wǒ (suǒ )gào sù nǐ de nà gè mì mì ，qǐng nǐ bú yào gào sù bié rén 。
Please do not tell others the secret I told you.
[1] Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.
