This manual is designed to assist teachers of adult education in the task of modifying commercially available ESL materials as a means of providing their students with sufficient, meaningful practice. The manual is divided into two parts. Part 1 is an introduction to the basic principles underlying sentence structure. This theory is considered essential for teaching the language effectively. Part 2 presupposes a working knowlege of the theory presented in Part 1, and provides practice in the application of the theories to the process of modifying typical ESL materials. A companion publication, "Placement Guide to Accompany 'Modifying English as a Second Language Materials,'" contains objectives for the manual, quizzes to measure the degree to which objectives have been realized, and means of locating the reader's appropriate point of entry into the course of study. It also allows the package to be used as a programmed self-study course. (Author/AM)