Sentences in Chinese may be divided into two basic categories: subject-predicate and topic-comment. Subject-predicate sentences are generally used for narrative purposes, whereas topic-comment sentences provide description or comment. The former essentially express an action, while the latter express a state:
Tā men zuó tiān qù diào yú。
They went fishing yesterday.
(他们tā men subject, 去钓鱼qù diào yú predicate)
Tā men de ài hǎo shì diào yú。
Their hobby is fishing.
(他们的爱好tā men de ài hǎo topic, 是钓鱼shì diào yú comment)
 Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.