Also, like many, you've likely forgotten much of what you learned. Where does the comma go, for instance? Is ending your sentences with a proposition really that bad a practice? Are there hard and fast rules for when to use who, that or which? All of these questions, as well as many more, can plague both amateur and professional writers. Doing a grammar and spell check helps to settle these questions and give you more confidence in your writing.

Check out http://www.grammarcheck.net/. It's totally free, checks for grammar and spelling mistakes, and gives instant feedback.