In many parts of the world, learners of English as a Foreign Language (EFL) or English as a Second Language (ESL) often face language difficulties and challenges in their performance in learning. These challenges and errors have multiple forms and causes, covering various language and skills aspects. This study focuses on the types of grammatical errors made by Foundation Programme students at a university in Oman. The study specifically examined areas that appeared difficult for the students and their actual knowledge of grammar. To serve its objective, the study collected data from the results of two grammar quizzes in ongoing assessments (OA) by 83 undergraduate students enrolled in a Foundation Programme studying English courses. Findings reveal frequent errors performed by students in certain grammatical aspects and language use. Results also show the significance of analyzing students' performance in OA for a better understanding of their level of language knowledge. The study also drew some pedagogical implications.