The current study tests the Interface Hypothesis through forward and backward anaphora in complex sentences with temporal subordinate clauses in highly proficient English-speaking learners' second-language (L2) Chinese. Forward anaphora is involved when the overt pronoun "ta" "he/she" or a null element appears in the subject position of the main clause, whereas backward anaphora is involved when it is in the subject position of the temporal clause, because the main clause always follows the temporal clause in Chinese. Specifically, the article tests the syntactic and discourse constraints in the interpretation and representation of "ta" and the null element in complex sentences. "Ta" is constrained by the syntactic cyclic-c-command condition. Thus it is possible for "ta" to refer to the other sentential subject in forward anaphora, but not in backward anaphora in Chinese. Unlike English, Chinese allows a null element in subject positions of finite subordinate and main clauses. It is proposed in the article that the null element in these positions is a Ø[subscript topic], a syntax-discourse interface category. Results from an acceptability judgement task and a picture judgment task indicate that Ø[subscript topic] at the external interface has been acquired, whereas the cyclic-c-command condition within narrow syntax is fossilized in L2 Chinese.