An Earful of English
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The name "Nguyen" is the Vietnamese spelling of a name that is difficult for most Westerners to pronounce. To learn how to pronounce this, consider two parts: The hardest part is the "Ng" at the beginning. This is the sound that English speakers use at the end of words like "sing," and "king." The second part is easy; it is pronounced pretty much the same as the word "win".
And, if this is the best you can do, go ahead, and call them "Mr. Win," and "Mrs. Win." It sounds much better than "Nu-goo-yen!"
However, one step better than that would be to put an "N" in front of the word "win", to make something more or less like "Nwin." That is almost perfect.
Finally, if you want to really, really get it 100% correct, practice the following:
Ngwin. Nguyen. Ngwin.