While breathing (and the muscles of breathing--see last week) is critical to good singing, another powerful aid to good singing is the proper use of the tongue.
In general, the following should happen when you are singing any vowel: The tip of the tongue is lightly resting against the back of the lower teeth.
It doesn't matter whether it is "ah" or "oh" or "ee" or "ooh" the tongue is lightly planted against the back of the teeth.
In American English (as opposed to Dr. Tim's British) the consonants are a great impediment to the proper use of the tongue. They are consistently working against a "forward tongue."
When you are finished with tonight's rehearsal, go home and watch yourself sing in a well-lit room with a good mirror. Sing the following:
Nah..........
Noh..........
Nee..........
Noooh.......
Try to keep the tongue consistently forward! Please work on this...we'll have more next week.