This study investigated the relative clause formation and the coding of tense in the English interlanguage of thirty-two Palestinian students at the six colleges of the Islamic University of Gaza.Three composition topics were designed to elicit the learners' expression of the various English tenses and relative clauses.Findings: The data show that Palestinian learners' switching of tense results from using English morphology to express an aspectual system similar to that of Palestinian Arabic. The English past tense and present tense are used to mark Palestinian perfective and imperfective aspects, respectively. The subject-verb agreement marker and the concord markers 's/is and 'm/am are omitted in relative clauses and when there is a change in aspect.In the area of relative clauses, the data show that relative clauses are ninety percent independent of the Palestinian Arabic structuring of relative clauses. The learners use resumptive pronouns not only in clauses where the predicate incorporates a noun, adjective, or a prepositional phrase, but also before verbs. Conclusions:1. There is a large amount of influence from the Palestinian aspectual system on the learners' use of English tense.2. Subject-verb agreement problems can be solved when the problem of tense shift is solved, because of the cooccurrence of these phenomena.3. EFL teachers in Gaza Strip should not over-react to their students' tense usage in narrative passages and should not require them to write in a particular tense in an artificial manner.4. The learners' errors should be tolerated and should not be considered as indications of faulty learning.