This study investigates the effects of motivation and structured input on the acquisition of English past tense regular forms. The role of motivation in the positive effects generated by processing instruction has been investigated in one previous study (Farhat & Benati, 2018). However, more research is needed to generalize the initial findings to a different language, language background, processing problem, and age. In addition, the present study measures the effects of the practice component of processing instruction, as no explicit information was given to the participants of this classroom experiment. The Academic Motivation Scale (AMS) questionnaire was used to capture different variables that influence motivation in order to create two different groups (highly- and nominally-motivated). In this experimental study, 43 native Chinese secondary school-age learners (L1 Mandarin) were assigned to three groups: a highly-motivated group (n = 16), a nominally-motivated group (n = 14), and a control group (n =13). The two instructional groups received structured input activities, which lasted for two hours. The control group received no instruction. A one-sentence-level interpretation test was used in a pre-test and post-test design to measure instructional effects on the interpretation of sentences containing past tense forms. The results indicated that both instructional groups improved equally from the pre-test to the post-test. Both groups outperformed the control group. Structured input was shown to be the main factor for learners' improvement in performance regardless of their level of motivation.