Abstarct: This study investigates the effects of 1) explicit information (EI) provided to learners prior to treatment and 2) individual differences (specifically working memory) within processing instruction (PI). Previous PI studies have all tended to provide the same explicit information on the past simple tense. The studies did not make learners aware of different allomorphs of -ed at the end of the past simple verbs (namely /t/, /d/ and /ǝd/ ). To investigate any possible effect of this awareness regarding variations in -ed allomorphs, ninety intermediate-proficiency learners were divided into one control group and two experimental groups: 1: processing instruction-conventional group (PIC) who were exposed to the explicit information provided in previous PI studies on the past simple tense (n = 30) and 2: processing instruction-focused (PIF) who received extra information regarding the three allomorphs of -ed at the end of the past simple tense morphemes (n =30). Two interpretation and two production tests were used in pre- and post-tests (immediate / delayed effects). Within the structured input activities (SI) component, learners were exposed to 7 interpretations and 3 affective activities. The processing problem in this study centered around the Lexical Preference Principle and the Preference for Nonredundancy Principle. All participants also took the PASAT (Paced auditory serial addition test) which measured their working memory capacity. Results from the Kruskal-Wallis Test revealed that both experimental groups improved in the post-test, however, the PIF group who received information regarding the phonological variations of the past tense verb endings demonstrated a more significant improvement. Furthermore, the working memory capacity of learners emerged as a critical factor, as individuals with higher working memory capacities consistently outperformed their counterparts with lower proficiency levels across both experimental groups in the interpretation tasks. Notably, the lower proficiency learners in the PIF group who received detailed explicit instruction regarding the phonological variations of of -ed exhibited a more substantial enhancement in the interpretation tasks. Subsequently, the result of delayed post-test revealed that the effect of PIF group remained over time.