Charles Darling's Guide to Grammar and Writing, maintained by the Capital Community College Foundation, is a useful resource for students. This particular page on subject/verb agreement includes twelve rules for recognizing and correcting errors in subject/verb agreement, a PowerPoint© explaining subject/verb agreement, a link to a chapter from an online textbook, and several quizzes to test students' understanding.
Link to Charles Darling's article on Subject/Verb Agreement
ENGL1010 Composition I