Adults should learn English mainly by using it, and it is best to attend training classes, so that they can force themselves to communicate with others. Read grammar books and words systematically and learn from others. What they say is generally authentic and popular! You can go to hiknow English to have a look! If the formal class is open to the public, you can attend it for free. You don't need to apply, and you can enter any class on your own during class hours.