This sound is used in words like zon (sun), onder (under) and van (from/of).
Pronunciation:
Chinese people often pronounce the n as a nasal sound. However, in Dutch the n is not a nasal sound (except when pronounced in the combination of a g, so ng).
First, place the tip of your tongue behind the upper front teeth. Then make sure that the sides of the tongue touch your right and left upper teeth. The sound you can produce now (the n) should be at the front of your mouth, not at the back of your mouth or through your nose. After practising this you can practise this sound in the combination of an o and a, so in on, an (or aan).
Now you should be able to pronounce the sound on (picture 1) and an (picture 2).