It's been a while I haven't posted on the forum, for personal reasons. I love to help people solving doubts mainly regarding grammar, and have found this post which remained without an answer.
I had a look around the net and found a good explanation here:http://random-idea-english.blogspot.co. ... th-do.html
Hope it helps!
Syl's English Corner - Learn English and Have Funhttp://www.ginnegar.0catch.com