“Punctuation marks are the primary means of showing the grammatical organization of what you write. Hide punctuation and hide grammatical structure. And if you hide the grammatical structure, you hide the meaning of what you're trying to say." Crystal (1996:151) argues. Since punctuation is relevant and necessary for grammar (as the quote above states), a significant cause of EFL learners' and Arabic EFL writers' poor punctuation proficiency is the misapplication of rules (Al-Jarf, 2001). . He highlights the incorrect application of rules caused by: 1) inability to master the operational or transformative component of a rule; and 2) inability to learn a domain of rule applicability (p.16). According to Mohammed (2006), EFL learners who are unable to write complete sentences and appear to constantly have difficulty understanding the concept of sentence; as a result their writing contains unconventional punctuation. Mann (2003) believes that one must be familiar with sentence boundaries in order to punctuate correctly; furthermore, he points out that starting and finishing sentences is one of the difficulties students encounter when they begin to write. Since sentence construction varies from one language to another, such variations can result in incomplete writing and therefore inappropriate use of punctuation. A simple sentence is “an independent clause that expresses an idea” (Al-Khresheh 2010, p. 106). , "a syntactic unit containing a finite verb" (Fischer 1984, p. 15, cited in Polio 1997, p. 107), composed of a subject-verb combination, although the subject can be compound and thus constitute a clause with “more than one constituent” (Al-Khresheh 2010, p. 106). These characteristics describe English s...... middle of paper ......O'Grady, et al. 1996, cited in Al-Khresheh 2010, p. 106). Othman (2007) places the differences in Arabic subordinate clauses at the helm of the differences and the cause of the impact on punctuation in English writing. Othman (2007) conducted a study in which he attempted to find out how subordination and coordination are commonly used in Arabic and English texts. He concludes: Subordination is seen as a sign of maturity and sophistication in English writing, while coordination is more commonly used in Arabic writing. As Mohamed and Omer (1999) also conclude, these differences, like those in coordination efforts, manifest themselves in different ways: Arabic subordinate clauses are semantically subordinate, but they are syntactically capable, like their main clauses, just like their main independent clauses, to act independently as separate sentences” (p. 293).