sub-grade in collapse zone

例句：

1. 1. The sub-grade students arestill learning the basics of mathematics.
   初级学生仍在学习数学的基础知识。
2. 2.The sub-grade teacher was very patientwith her students.
   这位初级教师对学生非常有耐心。
3. 3. The sub-grade level is designedto be easy for beginners to understand.
   这个级别是为初学者设计的，易于理解。