PunctuationLinks and learning materials covering English punctuation rules
Strictly speaking, I suppose punctuation does not fall into the category of grammar, but for website design purposes it is simpler to put this page here.
General rules of punctuation: The Online Writing Lab (OWL) on Purdue University’s website has a lot of helpful resources for writers and people interested in improving their understanding of English grammar and usage. Its Brief Overview of Punctuation page is a fairly basic but solid introduction to general rules of punctuation. OWL also has a Basic Punctuation Exercise and a Quotation Marks Exercise so you can practice applying the rules explained on this page.
Commas: Correct use of commas presents some difficulty for a lot of people. Once again, I highly recommend the explanations and exercises available on OWL:
For a more extended treatment of specific comma-related issues, refer to these pages on OWL:
OWL also has extensive exercises on comma use.
Apostrophes: Difficulty with apostrophes (in particular, using apostrophes in plural nouns) is quite common, but most people can learn to correct these common mistakes fairly easily when the rules are spelled out for them. See OWL’s The Apostrophe handout for a clear explanation of contemporary rules about apostrophe use. (I say “contemporary” because some of today’s standards are different from traditional standards.) There is also an accompanying apostrophe exercise.
My extensive and carefully researched List of Common Errors in English Usage handout has an explanation of the differences between “its”/”whose” and “it’s”/”who’s.”
Hyphens: See OWL’s Hyphen Use handout.