Chinese is a topic-prominent language. Therefore, sometimes the subject can be omitted, which is represented in the syntax as zero-subject structures or in the phrase of traditional grammar, "non-SVO sentence", which believes that the subject is not essential to restore or cannot restore (Huang and Liao 2002 as cited in Yang, 2014). According to Generalized Case Filter (GCF) proposed by Xu (1993), the nominative Case assigner in Chinese is optional, making it acceptable for the assigner not to use its assigning ability in the null subject structure, in which the subject's position is empty. For instance, "他每天都這樣，只吃快餐，又不做運動，常常又晚睡。" (He is always like that. He only eats fast food. He also doesn't exercise. He often sleeps late.) We can see that the subject "He" is omitted in the last four clauses. As the deleted constituent is typically coreferential with the topic in topic-prominent language, the "他" (He) is the sentence's topic. It is grammatically acceptable, and the omission of the subject/topic does not affect the meaning conveyed. However, the topic should be mentioned at least once at the beginning of the conversation or recognized by the hearers first to avoid confusion. For example, starting with 他喜歡男人 and 喜歡男人 can have two meanings. Both can be understood as "he loves men.", but the second one can also be read as "I love men."
On the other hand, the object can also be omitted. It is reflected in the transitive verb in the syntax without a complement (Yang, 2014). For instance, 喝不喝？, meaning “Do you wanna drink (the wine)?”. The argument 酒 is needed, but it is omitted. Regarding the GCF, Chinese transitive verbs can have their Case-assigning power discharged or left un-discharged, both of which are acceptable grammar choices.
Reference:
Xu, J. (2003). Sentence Head and Sentence Structure: A Study with Special Reference to Chinese. Longman.
Yang, Y. (2014). Generalized Case Theory and the Argument-Omission Structure in Mandarin Chinese. Chinese Lexical Semantics, 441–447. https://doi.org/10.1007/978-3-319-14331-6_44
