I could correctly pronounce 'da' 'de' pairs of letters, as many native speakers told me:
just put the tip of your tongue between the upper and lower teeth and against the back of front teeth.
After training, I could pronounce them. However, I find another problem that when I speak a whole sentence, I always bite my teeth when I pronounce 'd' in the sentence, especially when I speak it quickly.
It is so unnatural to pronounce this sound. I don't believe native speakers have the same problems.
There should have some methods to speak 'd' when speaking fast, can anyone give me some tips/suggestions?