This study investigated common written errors and the causes that language learners frequently commit. A descriptive qualitative approach was conducted with the participation of 57 eleventh-graders at a high school in Hung Yen province during the second term of the academic year 2021-2022. The respondents were requested to participate in five writing tests in three successive months to generate their morphological, lexical, syntactical, and mechanical written errors basing on Ferris' (2014) model of Error Analysis. The results reveal that the respondents are not good at English writing due to both the negligence of the testing and assessment format, which concentrates on multiple-choice tests and the demotivation and discouragement of learner autonomy in improving writing skills. The finding of this research would help the educational policy-makers adjust English teaching curriculum to give a priority to promote writing skills. Simultaneously, teachers of English should change their pedagogical practices to raise their learners' awareness of the key role of error correction in writing skills. For English language learners, they could recognize the importance of corrective feedbacks from their teachers, peer correction on their written errors that they committed while learning to write effectively in English. Lastly, future studies might confer the findings of this research for the reference to serve as the resourceful enrichment of their research liability.