Language learners come to class with their own phonetic rules. In the contemporary setting, the teacher is going to have to deal with a variety of sounds and different accents. I would not dwell too long on the phonetics though. Instead I would try to integrate that gently into the overall lesson so that the learner concentrates on other things that will help him to recognize and apply the new sounds. Don't expect the learner to learn swiftly if at all, especially if his language roots are completely different.
If there is a difference in the sounds heard try and get the student to repeat the new sound but I would that through examples. Often enough the student cannot pronounce the difficult sounding word on his own and needs to hear words that are similarly sounded out. In the case of pronouncing the past ed for regular verbs it is standard for me to have them hear the harder d sound in "ed" ending words that have a "d" or "t" before the regular ending. The student can compare this sound with other "ed" endings found in words having other consonants. Similarly the student can practice the different "s" sounds that occur with plurals or when pronouncing the present form of a verb in the third person. Some sound more like "z" and this happens for certain words that end with an "ch" in the singular. Problems arise when the student has to differentiate between long and short vowel sounds and the teacher can list similarly structured words that have a short vowel sound like "mit" and then add an e to the ending of the word on the right hand side of the page. The student will make a connection between the changed structure of the word and the change of the vowel sound from short to long.
Doing phonetic exercises is then useful and the ones mentioned are only a few of what the teacher can do for the student.
