عنوان مقاله [English]
This study aims at investigating the structure of the written discourse of mentally retarded students at prevocational level, according to MLU, different types of processes, and themes. The theoretical framework of the study is the experiential and textual meta-functions introduced in Systematic Functional Grammar. The informants are 120 mentally retarded students in Tehran, who have been chosen through a stratified random sampling. A test including 4 picture stories was administered, and as a result 120 writing samples including 1110 clauses were collected. The number of clauses increases as the grades increase. MLU in first, second and third grades of prevocational level, is reported to be 4.33, 4.79 and 4.66. The distribution of different processes and different themes are as follows: material (56%), behavioral (13%), relational (10%), verbal and mental (9%), and existential (less than 1%). Subject theme (43%), Null Subject theme(30%), textual theme(23%), circumstantial theme(2%) , and other types less than 2%. The frequency of material, relational and verbal decreases as the grade increases while for mental and behavioral, increase in frequency is observed. In the second grade, the highest MLU and textual theme frequency is reported. Complex theme is observed in about 30 percent of clauses.