The subject of a sentence is generally of a definite nature; i.e. it is a specifi c entity that is known to both the speaker and the listener.
昨天王老师来我家。
Zuó tiān wáng lǎo shī lái wǒ jiā。
Yesterday Teacher Wang came to my house.
(王老师wáng lǎo shī is definite.)
Improper:
昨天一个客人来我家。
Zuó tiān yī gè kè rén lái wǒ jiā。
(一个客人yī gè kè rén is indefinite.)
Correct:
昨天我家来了一个客人。
Zuó tiān wǒ jiā lái le yī gè kè rén。
Yesterday a guest came to my house.
[1] Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.
[2] Teng, W. H. (2016). Yufa! A practical guide to Mandarin Chinese grammar. Taylor & Francis.
