PHONOLOGY: "The Schwa": The most important sound in English.

    "Schwa" is the name of the most common sound in English. It is a weak, unstressed sound and it occurs in many words. It is often the sound in grammar words such as articles and prepositions.  Getting the schwa sound correct is a good way of making your pronunciation more accurate and natural. 
The phonemic symbol for this sound is /Ә/.
     In unstressed syllables

Any vowel letter can be pronounced as schwa and the pronunciation of a vowel letter can change depending on whether the syllable in which it occurs is stressed or not. 

In the word 'man' the letter 'a' has its full sound - represented by the symbol /æ/.  In 'postman' the syllable 'man' is not stressed and the letter 'a' is pronounced as schwa, represented by the symbol .

          Not just a letter

The sound schwa does not only represent a single letter. In some words it is the sound of several letters or even a whole syllable. This is often, but not only, seen in words which have a syllable made up of a vowel letter followed by the letter 'r'. Remember the schwa sound is only used if the syllable which it is in is not stressed. 

Find out more:

http://www.bbc.co.uk/worldservice/learningenglish/grammar/pron/sounds/vowel_short_5.shtml