As a French and Spanish Professor for many years I was always alert to second-
language acquisition
linguistic differences.
In my undergraduate years as a student the emphasis was not totally in
language production as much as in translation.
Where I ran pell-mell into my native Spanish sounds vs. my second language acquisition potential problems, in this case, French was at a Total Immersion French Institute in Colorado. It was pointed to me that my frontal French "u" sounded like a Spanish back, "ou". This was all accomplished very diplomatically by one French professor who took me aside and recorded my "u" production. To my dismay I actually heard it over and over again as a Spanish sound, "ou".
The next course of action was to empahsize the formation of the French frontal "u" sound which is produced at the front of the mouth with the lips rounded as if to say the "oo" in the word "moon" and, instead, pronounce a constricted "ee" sound as in the word "feet". This must all be done in one sound at the front of the mouth with pursed lips so as to isolate it in the front area and not at the back of the throat as in the back "ou" sound in Spanish.
The practice examples that I recall most vividly were inscribed in the sentence: "La russe est rousse"(The Russian lady is red-headed). The emphasis was in the single "u" of russe which was the frontal "u" in question versus the back "ou" of rousse. It was, of couse, encouraged to switch back and forth to stress the change of meaning depending on the single "u" in contrast to the back "ou" of the back of the throat Spanish sound.
It was , indeed, most revealing early on to have discovered that I was producing Spanish sounds instead of the French, but equally as important to know how to correct the
linguistic phenomenon in myself as a native Spanish speaker to best help myself and my future students in ensuing years.