A regular program of study is the general program that the vast majority of students in the program will follow. For example, all students will take all the same courses.
A modified program of study will deviate from the regular program. There will be differences from the regular program. For example. some of the courses may be different; the length of the program may be different, etc. A regular program may be modified to meet the needs of a particular student.
Teaching ESL to Adults