M. A. K. Halliday made quite clear his point of view that linguistic events should be accounted for at three primary levels: substance, _____, and context.
【正确答案】：form
【题目解析】：韩礼德认为语言材料可以在三个层次上进行解释，实体、形式和语境。实体指语言原材料，即声音上或书写上的符号；形式指把实体排列成有意义的结构，包括语法和词汇；语境是语言出现的场合。所以，本题填form。