Purdue University has a great website dedicated to writing, but is also an excellent source for grammar tips. It also has practice worksheets with answers for personal or classroom growth. Without answers, or corrections, a person can get stuck using bad grammar while thinking that it is correct. Remember that language encompasses reading, writing, speaking and listening. Practice language in each of these four areas (with corrections) and your abilities will grow. Speaking with a native speaker who will correct your speech is a humbling, but great way to improve speaking and listening skills. Another way is to memorize poetry or sections from your favorite story that have correct grammar in them. Those memorized lines will help you to notice sentence structure and word order. It also helps when shifting from reading to speaking because those memorized lines become a foundation for your own speech.
I believe the best way to improve anyone's grammar is to read good writing and to read lots of it for years. It is wise to subscribe to the best periodicals such as the New York Times and the New Yorker magazine. Naturally it is helpful to talk with people who use good grammar, but this is not always possible, whereas you can always read interesting mateerial in good newspapers and magazines and be absorbiing good grammar at the same time. Not all books contain good grammar, but nonfiction books published by the best publishers will invariably contain good grammar. This is the way to acquire good grammar without studying a lot of rules.