Two sounds close enough to trade places: the “th” of this and the “v” of van. Swap one for the other and a different word comes out — this page is how to keep them apart, in both directions.
Take your top teeth off your bottom lip and place your tongue tip between your teeth, keeping the voice buzzing.
“The lip auditioned for the tongue's role again! Basta! Tongue between the teeth, keep the buzz — ecco, perfetto!”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.