I think Harry is right about the thread idea. I haven't seen an enormous number of grammar issues here, not enough for a full forum, but I have no doubt that there are people who, like you, have questions to ask. Incidentally, as one of the resident "grammar Nazis" I'll be happy to answer questions in such a thread.
Aside from that, the Web is full of sites eager to help improve one's grammar. For a start, I would recommend
Grammar Slammer and
Common Errors in English. I wouldn't say they're the best websites out there, nor even necessarily the most comprehensive, but they do at least provide somewhere to begin.