Two sounds close enough to trade places: the “th” of this and the “d” of dog. Swap one for the other and a different word comes out — this page is how to keep them apart, in both directions.
Instead of tapping the ridge, slide your tongue tip between your teeth and keep voiced air flowing without stopping.
“D is a knock on the door — this sound is the door left open! Tongue between teeth, keep the voice humming through.”Nunzio, when he hears it
You cannot hear your own swap. That is the whole problem.
Nunzio records you reading a sentence, scores every sound in it on your phone — nothing is uploaded — and tells you which of these two actually came out. Nunzio — Accent Coach is coming to the App Store and Google Play.