This post is copied and revised from my ICTY English course blog at http://ictyenglish.blogspot.com/. At this site you will find links to online resources on grammar including interactive erercises).
Writing is not all about grammar. It's also about understanding situations, relating to people, and knowing what you want to say. However, having a wide repertoire of grammatical techniques can help you to write in a more flexible and effective way.
As English becomes more of a diverse and international language it is less and less possible to point to an authority and say 'that is the correct way!' (or 'that is a mistake!'). But there are generally agreed conventions for British and American English grammar which it is helpful to know, especially when you write.
For reference, I would recommend Advanced Grammar in Use (Second edition 2005) by Martin Hewings (Cambridge University Press, ISBN 0521532914 with answers). This book covers grammatical points in an imaginative way, giving examples and practice exercises on facing pages. Also has useful summaries at the back. If you buy this book for self-study, make sure you get the one with the answer key! The third edition of this book also has a CD Rom. There is an Intermediate level English Grammar in Use by the same author.
For more practice exercises, try Advanced Language Practice (Second edition 2004) by Michael Vince (Macmillan Heinemann, http://www.macmillaneducation.com/ , ISBN 0435241249 with answers).
Online Grammar Resources
My top recommendation is the BBC Learning English site at http://www.bbc.co.uk/worldservice/learningenglish/index.shtml. This is an excellent resource for reading, grammar and vocabulary - updated every day. Always worth a visit.
I would also recommend the OWL (Online Writing Lab) at Purdue University for guidance on grammar and writing: http://owl.english.purdue.edu/handouts/grammar/index.html.
For a quick at-a-glance summary of the main tenses in English, see my Yahoo page here. (Drag the cursor over the tenses for examples)
For your questions on grammar try the Blue Book of Grammar at http://www.grammarbook.com/grammar/cnt_gram.asp or this FAQ site http://www.drgrammar.org/faqs/ and Paul Brian's Common Errors site at thttp://www.wsu.edu:8080/~brians/errors/errors.html.
For questions on usage you might try the vast Alt. English Usage at http://alt-usage-english.org/index.shtml. This is searchable and very good for settling disputes over usage, such as: 'Do we say different from, different to or different than?'
You should be able to find answers to most of your questions on these sites. If you can't, email me and I will see if I can find an answer for you!
Practice Online
For quick practice exercises and tests on most areas of grammar see http://www.world-english.org/.
For some useful quizzes on English grammar, donated by teachers, see http://a4esl.org/ (uses mostly Java and Flash). Also see the bilingual quizzes here.
I would also strongly recommend the pages on tenses at Englishpage.com. All the tenses are explained clearly and there are online practice exercises for each one. See http://www.englishpage.com/verbpage/verbtenseintro.html.
A teacher, David Tillyer, has produced a useful summary of a tricky area - gerunds and infinitives - at http://www.geocities.com/gwyni_99/gerinfless.html. He also offers a list of verbs and practice tests.
For articles, see my own site at http://articlespace.blog-o-matic.com/.
