Two sounds close enough to trade places: the “ur” of bird and the “eh” of ten. Swap one for the other and a different word comes out — this page is how to keep them apart, in both directions.
Keep your lips neutral, tongue in the middle of the mouth, and hold a long central vowel with no fronting.
“Come back to the center, bello! Not front, not back — the tongue rests in the middle, lips relaxed, and hold it: errr.”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.