The links below are free and reliable. They will be available even after this course ends, so you would be wise to bookmark them.
This link to the CCC Index page will take you to discussions of all the basics, The site includes exercises and an easy-to-miss INDEX link (rectangular box in the center) which itemizes 427 specific items in the site. Unfortunately, this site includes advertisements.
This page of English Grammar Rules focuses on punctuation. It also includes self-tests so you can see if you really understood the item.
The Excelsior OWL is very process-oriented, and includes a special section to help you with reading skills.
This link to the UNC Writing Lab is especially rich in help for writing in a variety of situations. Some of the material in the lower right is specific to UNC, but almost all the rest can apply to us.
The Purdue OWL (Online Writing Lab) is legendary. (Note: You will run into advertisements. Just ignore them.)
This is a fill-in-the-blanks website which will generate MLA Works Cited entries and APA Reference page entries.
I do not have much experience with this site yet, but it looks very promising for generating bibliographic pages in a wide variety of formats: MyBib. It does not appear to give any help for making the in-text citations.
These dozen or so pages take up topics which seem to cause trouble to the majority of my students.