Freshman students at the College of Languages and Translation received direct instruction in plural formation. Instruction covered regular plural nouns, irregular plural nouns, plural formation of words ending in -f, and -o, nouns that have the same plural and singular form, and words with Latin and foreign plurals. The students did all the exercises in the textbook, then took an immediate test a week after instruction and a delayed test at the end of the semester (3 months later). Responses were scored and a corpus of 3099 errors was collected from both tests. No significant differences were found in the amount and types of errors made by the students in the immediate and delayed tests. Results revealed that freshman students tended to regularize English plural formation and overgeneralize regular English plural morphemes (63.28%), i.e., they deleted the regular plural suffix from nouns ending in an -s or -es (35.37%) or tended to add the regular plural suffix to words that do not have it (27.91%). They also confused singular and plural endings of Latin words (15.07%). They either confused the singular and plural forms of the same Latin word or added a faulty Latin suffix to a Latin word or even a non-Latin word such as "criterium," "*curriculon," "*natia," "*salma," "*petrolea." In other cases, they thought the singular and plural forms of a word were the same (7%). In addition, findings showed that the most difficult plurals to master were those of words that end with an -s or -es but have no singular form such as measles, news, pajamas, means, linguistics (28.85%); words with Latin plurals (21.85%); non-count nouns such as "information," "electricity," "petroleum," "salmon" with no plural form (21.4%), and words that have a plural, but they thought they have no plural form such as "nation," "illness," "infection," "African" (8.55%). Interference among the English plural morphemes themselves and confusing plural formation rules caused most errors. No interference from Arabic pluralization was found. Recommendations for improving students' English plural formation competence are given.