Consider where your current skill level is. This can be affected by various factors. Is English a second language for you? Are you a beginner to grammar education, or have you had previous education? Who are the classes for--an adult or a child?
The educational site Edufind.com provides users with English grammar tests that you can use to evaluate your language skills. This will help you determine what areas you need to work on. This information will assist you when you review educational resources.
Are you looking to take entire online classes or do you just need assistance on occasion to look up a specific grammar rule? There are sites for either specialty.
Consider what your personal learning style is. Are you a visual learner? Do you do best being able to listen to your lessons? Remember to consider this when evaluating which online lesson to select.
AgendaWeb.org is a great resource for those times when you need to get an answer to a grammar question. The site lists the various parts of grammar as links that you can click on for immediate explanations.
Universalclass.com offers a wide variety of English grammar classes online at different levels of expertise to suit various students' needs. While this is not a free site, successful completion of classes from this site rewards users with Continuing Education Units, which can be useful for professional purposes.
If you are learning for your own personal edification, free sites such as Tolearnenglish.com may be ideal. This site does not offer Continuing Education Units, but it will help you learn English grammar skills and assess your progress. It also offers games to put some fun into your learning experience.
Englishpage.com describes itself as "a grammar book of high-quality English lessons and exercises to help English learners learn English grammar online." The site collects recommended lessons that users have found helpful. This service is free.
Exercises will help you reinforce the lessons you are learning. Many sites offer English grammar exercises.
One example of such a site is ToLearnEnglish.com. This is a free site where you can practice your skills.
If you would like exercises that you can print out and work on, Autoenglish.org offers many of these for free.
Another idea is to find an English language forum where you can discuss and ask questions of others seeking to improve their skills, too. Englisch-hilfen.de has such a forum. Members may be able to offer suggestions for online grammar exercises they have found helpful.