The Jehovah’s Witnesses use their own translation of the Bible.  Unfortunately they wrongly changed many things to make it fit their theology.  Not only have they made a bad translation, they deny any other translation as valid or inspired.  Even more unfortunate is that they do not have Greek scholars, if they did they would find many errors in the New World Translation.

This would be like me taking a comic book and using it as my Bible.  Then, denying that anything else is correct to any degree.  It is quite ignorant and can be frustrating when talking to a JW.

I would like to point you to an article from Christian Apologetics & Research Ministry.  I hope you can find some good information.

 http://www.carm.org/jw/nwt.htm