Typically, lectures will provide only a very general overview of the principles and concepts presented in much greater detail in the reading assignment. Lectures are a vehicle for presenting the newest findings on topics, for presenting alternate perspectives, for considering applications of the material and for raising values issues. Lectures need to accomplish quite a lot in addition to summarizing material in the textbook. Your textbook should be viewed as a kind of gateway into a new body of knowledge. You are responsible for learning this material in your textbook and also material presented in lectures.
Textbooks are highly condensed information.
It takes active effort to assimilate the information presented in the textbook. Applying optimal study methods such as using the SQ4R and the Cornell note taking system will facilitate learning that material in the textbook. The work load for a college course is a function of the number of academic credits which will be awarded for the course. By long standing tradition, faculty presume that students will work two hours outside of class each week for every academic credit they receive for a course. If you that you are having difficulty keeping up with the workload for your courses, the first question you need to ask yourself is whether you are really putting in 25-30 hours each week studying in addition to attending lectures.