Schwa is the name for the most frequently used vowel sound in English. It is used for Short-u, the alternate Short-o, and reduced vowels.
The Short-u sound is in many words that are spelled with a “U”, such as: fun, up, just, much, under, bug, shut, must, such, us, but, luck, mud, number, rush, judge, truck, deduct.
The letter “O” often borrows the Short-u sound, especially in frequently used words. For example: love, month, some, done, from, of, son, front, among, other, nothing, none, wonder, does, mother, come.
Schwa is the sound that any vowel letter can take in an unstressed (or weak) syllable.
“A” — In the word “ago”, the stress is on the 2nd syllable, so the letter “A” is in the weak or unstressed syllable. So instead of sounding like Long-A or Short-a, it becomes schwa.
“E” — In the word “system” the stress is on the 1st syllable, so the letter “E” sounds like schwa. In the word “before” the 2nd syllable is stressed, so the “E” in the 1st syllable becomes schwa.
“I” — In the word “pencil” the 1st syllable is stressed, leaving the “I” in the unstressed syllable, so it sounds like schwa.
“O” — In the word “second” the stress is on the 1st syllable, so the letter “O” takes the schwa sound.
It would be hard to say very much in English without using the schwa sound. The good news is that it is the easiest vowel sound to make! If you’re not sure how to say it, The Sound of Schwa gives an explanation.