Students are required to master effective sentences to support their writing skills. However, students often struggle with comprehending and constructing effective sentences due to their limited proficiency and competence. This research examines the comprehension of effective sentences through grammaticality judgment tests and its implications for writing instruction. This research employs mixed methods. Two hundred three first-semester students from Universitas Muhammadiyah Malang were selected for research using purposive sampling. Data was collected using grammaticality judgment tests and semi-structured interviews. Data analysis was carried out quantitatively using the Mann-Whitney U Test and qualitatively to describe patterns of findings. The results showed that 57% of students comprehended quite well, and 25% comprehended. Learners' comprehension of effective sentences is predominantly concerned with language efficiency rather than grammatical structure and lack of explicit linguistic knowledge. Effective sentences, especially parallelism, clarity, and explicit linguistic knowledge, are partially understood. The Mann-Whitney U Test shows that first- and second-language students comprehend effective sentences identically. Furthermore, male and female learners are dissimilar in their ability to comprehend effective sentences. Writing learning can emphasise mastery of effective sentences, explicit language knowledge, and collaborative writing based on language acquisition order and student gender. These findings can be applied to writing instruction by improving students' mastery of effective sentences in collaborative writing.