Many languages utilize some type of assimilation a change in a certain sound based on the sounds that come before or after it. One example of this is the difference in plural -s in "cats" versus "dogs". Pronounce each word while touching your throat. You should be able to feel that your vocal cords do not vibrate for the final -s in "cats", but they do vibrate for the final -s in "dogs", turning it into a z sound. This type of assimilation is widespread throughout the worlds languages. However, American English pronunciation also fails to assimilate sounds in some places where other languages do.

Another type of assimilation occurs when a consonant occurs before a u or o sound. (NB: sometimes the letters u and o represent other sounds, for instance the "uh" sound in "but" or the "ah" sound in "father".) These vowels are pronounced with the lips rounded. In many languages, consonants before these vowels will also be pronounced with the lips rounded. However, in American English pronunciation this is not the case. Set up your lips and tongue to pronounce the word "do". You should notice that your lips are not rounded or pursed. If they are, relax them. One way to practice this pronunciation is to insert another vowel between the d sound and the oo sound in "do". The extra vowel should sound something like the u in "but" or the a in "about". Once you are able to pronounce the d without the lip rounding, get rid of the extra vowel. Working with this advice will help improve your pronunciation.

Ryan Denzer King, Article to help English Speaking and English Pronunciation by PronouncePro American English Pronunciation http://www.pronouncepro.com/ Writing Staff.