Two sounds close enough to trade places: the “th” of think and the “f” of fun. Swap one for the other and a different word comes out — this page is how to keep them apart, in both directions.
Move your top teeth off your bottom lip; instead put your tongue tip between your teeth and blow.
“The lip is doing the tongue's job! Lazy tongue! Send the lip home, put the tongue between the teeth. Ecco!”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.