So I also asked a new question on stackexchange. My students have all started to say things like "Do you can play tennis?" Or "Where went you last night". Over- and underuse of 'do' in English! I asked for a little help figuring out why these were wrong and why we use the 'do-formation' in English. Well, I got way more than I needed! Lots of people went so far as to explain the technical linguistic reasons! Which, I must admit, reminded me why I fell in love with Linguistics in University...and makes me desperately want to study it some more!
However, I ended up choosing the answer that explained, quite succinctly I thought, the reason we need or don't need to use 'do' in English.
P.S If you noticed the link on succinctly I highly recommend that website for definitions! They provide definitions in increasing levels of English!