Two sounds close enough to trade places: the “th” of think and the “s” of sun. Swap one for the other and a different word comes out — this page is how to keep them apart, in both directions.
Put the tip of your tongue lightly between your teeth and blow air over it, no voice.
“Madonna! Your tongue hides behind the teeth like a shy cat. Let it peek out, just a little — ecco, now blow!”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.