It looks like you’re referencing the well-known book American Accent Training by Ann Cook (often subtitled A Guide to Speaking and Pronouncing American English for Anyone Who Speaks English as a Second Language ).
| Sound | Common error | American model | |--------|--------------|----------------| | /r/ vs /l/ | mixing (Asian languages) | = tongue mid, no touch; l = tongue tip touches roof | | /θ/ (thin) vs /ð/ (then) | replaced by /t,d,s,z/ | tongue between teeth, blow air | | /i/ (beat) vs /ɪ/ (bit) | same length/tension | /i/ is longer, higher, tongue tense; /ɪ/ relaxed | | /u/ (food) vs /ʊ/ (good) | /u/ too fronted | /u/ is back, lips rounded; /ʊ/ is lower, less rounded | American Accent Training A guide to speaking an...