A lot of foreign speakers talk with little mouth movement. In English, we move our mouths a lot! People have even told me that their mouths feel “tired” after an accent improvement session.
Several vowel sounds move. They include: “A”, “I”, “O” “U” and a vowel paired with an “R” (which is very similar to a vowel). These are LONG vowel sounds. When you don’t move your mouth the vowels sound very different and too short.
To correct this, make sure to move your mouth for these vowels. Watch others to learn which vowels move.
Good luck…people will notice the difference.