How intonation depends on correct vowels
An exclusive movement of the second formant is obtained by restricting yourself to the vocal sequence I-Y-U, resting the anterior part of the tongue, jaw and lips.
Let the tongue hang out of the mouth and place it loosely on the lower lip. Then sing English YOU and French OUI in slow motion. You will hear a soft sequence of overtones that both sharpen the ear and train the motor functions for the corresponding movements of the tongue base and epiglottis.
After just one or two hours of practice, experienced singers can position the second formant exactly to a semitone.