The prevalence of topic-comment structures is one of the most distinctive features of Mandarin Chinese. This type of sentences begin with a nominal representing a topic that refers to something about which a speaker assumes the listener has some knowledge. The rest of the sentence is a description of the topic, commonly known as a comment.
→ 我吃蘋果 (w˘o ch¯ı p´ınggu˘o/ I eat apple.)
This sentence has a normal Chinese word order, with the agent in front of the verb and the affected undergoer after the verb. However, 蘋果 (Píngguǒ) can be fronted to take up the topic position without any additional marking by the process of topicalization.
→ 蘋果我吃 (píng guǒ wǒ chī/ Apple, I do eat.)
Reference: Chinese: a linguistic introduction by Chao Fen Sun