I have written an e-Book about this and other problems students experience. You can get a free copy of the e-book at the Doctor English website by copying the link below and pasting it into your browser.
http://web.doctorenglish.ch/fc.asp?nav=fc&IDR=422
I also offer an online English course that can really help you to improve your speaking skills quickly.
You can find the information by visiting the webpage below: