The purpose of this study was to investigate the understanding of basic knowledge of mathematics for $6^{th}$ grade students in elementary school by an essay test and provide instructional suggestions for teachers. A total of 132 students from 6 classes in 3 elementary schools were tested and analyzed in terms of the characteristics of correct answers and types of incorrect answers. The results showed that students had poor understanding of basic conceptual concepts and principles throughout six content areas of school mathematics curriculum, despite their good performance on mathematical skills. This study included implications to teaching and learning for each of the content areas.