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Illinois State U.: Punctuation Made Simple Apostrophes
What is an apostrophe and how is it used properly in punctuation? "Punctuation Made Simple," gives some concrete examples for apostrophe use.
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Using english.com: English Grammar Quizzes & Worksheets
A nice treasure trove of printable grammar worksheets organized by proficiency level: beginner, intermediate, and advanced. Emphasis is on parts of speech but you'll also find some that focus on common spelling issues and vocabulary...