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...

This site has examples of some common mistakes when forming questions using the present simple verb tense. The correct form is given along with some of the more common errors.

Troublesome verb forms using the present simple affirmative for ESL speakers/writers. Examples of the incorrect use of the verb along with corrected form.

Examples of some troublesome verb forms using the present simple negative.