This study examined the effects of working memory (WM), structure, and salience on the processing of English relative clauses (RCs) and participial reduced relative clauses (PRRCs) by L1 Thai learners. Salience in this research is the phonological alterations required for irregular verbs to inflect into the past participial form. The study included two types of irregulars with different salience levels. Seventy advanced L1 Thai learners took a reading span task and a self-paced reading task to assess their WM level and processing of past participial forms. The hypotheses proposed that WM, structure, and salience would influence online processing (reading times) and offline processing (comprehension accuracy). However, the findings showed differences between the higher and lower WM groups in only their online processing. The asymmetrical WM effects may be due to different levels of resource demands of the two processing tasks. Effects of structure and salience were observed on the learners' processing. The learners read the PRRCs faster than the RCs due to greater processing resources required for considering grammatical issues in the latter. Additionally, the participants processed the less salient irregulars faster because of greater phonological similarities between their past tense and past participial forms. Future research could explore either different classes of past participles in the salience hierarchy or how WM and salience affect the PRRC processing among learners with diverse L1 backgrounds.