[openmind]

I think what helps is always to ask the other person what exactly they want to achieve. If their answer is "I want to learn English." then I ask them "Do you know how to learn English?" They usually don't. Then I ask them who they think might know how to learn English and they usually answer that I might be one of those people. Then is the time for me to tell them that they don't need to learn grammar rules and that it's much better to worry about the "why" question. If, however, they tell me that their purpose is to become a professor of linguistics then I tell them that they are in the wrong place.