Tongue tension is important for pronouncing English short vowels well. All of the short vowels in American English need a relaxed tongue. In fact, some books and dictionaries call these vowels “lax vowels”.

THE SECRET KEY for lax vowels

Most students of English do not seem to know about tongue tension. Many of my students have said that they were never told about relaxing the tongue. That’s why I call it the secret key.

THE CRITICAL FACTOR for some

For two vowels, Short-e and Short-i, tongue tension is critical. Failing to relax your tongue for these two vowels can make them sound more like a different vowel, which can cause misunderstandings.
Short-e can get confused with Long-A (see Sell or Sale?)
Short-i can get confused with Long-E (see This or These?)

Pronouncing the other Short vowels

Short-a-1 “man” “hat”
For this vowel, the tongue is low in the front of the mouth. The mouth needs to be open enough so that the tongue can go low enough, and with a relaxed tongue.

Short-a-2 “car” “ball”
Short-o-1 “hot” “stop”
These two vowels share the same sound. For this sound, the tongue is in the center, neither front nor back, and the tongue is low, so the mouth needs to be open. Think of saying “ah” for the doctor. The tongue is relaxed and the lips are not rounded. (this is compared to Short-u in Boss or bus?)

Short-o-2 “month” “son”
Short-u “fun” “duck”
These two vowels also share the same sound. For this sound, the tongue is completely relaxed in the middle of the mouth: neither front nor back, not high, not low, and the lips are not rounded. (this is the same as Schwa)

Short-oo “book” “good”
This vowel is pronounced in the same place in the mouth as the Long-U, as in “nuke”, but with a relaxed tongue, as in “nook”. This is the only Short vowel with rounded lips.  (see Short-oo?)

If you begin to relax your tongue for these vowels, you can improve the clarity of your pronunciation. Note: If relaxing your tongue seems difficult, think about relaxing it all the way back to the throat — the tongue muscle extends into the throat.