If I were to do a grammar tutorial, I would borrow, with the author's permission, heavily from it. The earlier quizzes are cgi-based while the more recent are done in JavaScript. A bare bones guide to English grammar. In spite of the title this is a serious site. As one might expect, many of the sample sentences have something to do with dogs.
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In addition, this site has one of the best selections of links to other grammar and writing related sites. I intend to make this section required reading for students next semester before they start their research projects. This is a classic example of hard to read text-garish colors of lime, dayglow orange, and pale blue against a black background. NWCA writing centers on-line. Since not all OWLS are created equal, this is a great site that allows the user to explore the content of hundreds of sites.