In the context ‘What did Zhang San do?’, the subject NP Z. S. conveys old information since he is mentioned in the question, and ‘cleaned the house’ conveys new information, since it is the information elicited by the question. In another context ‘Who cleaned the house’, then in the answer ‘Z. S. did’, Z. S. becomes new information and constitutes the focus of the sentence, whereas ‘clean the house’ is old information. Thus old/new relations are determined by the context; they are not stable but changeable.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.