In English, the vowel sound in a syllable often changes depending if the syllable is stressed or unstressed.  A useful starting point to learn about this is with a sound that sounds like ‘uh’ and is represented by two symbols in the IPA. It is the most common sound in English. It is ubiquitous because in most cases the vowel that is the syllable core changes to this sound when the syllable is unstressed. In unstressed syllables the sound is called a schwa and is represented by an upside down ‘e’ [ə].  

One place this happens frequently is the “-ed” suffix following a consonant such as ‘t,’ d,’ or ‘g.’  So we pronounce “waited” [weɪtəd] (WAY-tuhd), the short “uh” separates the ‘t’ sound from the ‘d’ sound. The stress is in the [eɪ] (ay) so the ‘e’ is unstressed and pronounced [ə] (uh).

The word “the” [ðə] (thuh) is not stressed, because it is a function word (next section).

The ‘uh’ sound can be a vowel in stressed syllables too. In that case it is represented by a the symbos [ʌ] which is called, “tuned v,” “caret,” “circumflex” and “wedge.” I am calling it ‘wedge” here.  Wedge [ʌ] is the symbol for the ‘uh’ sound in words like “but,” “cut,” “shut” which are almost always stressed

This applies in sentences too. If a syllable is unstressed in a sentence, the vowel is usually reduced to a schwa. Function words are not stressed, so the vowels in function words are usually reduced to schwa, as in “I’m going to the store” [aɪm ˈɡoʊɪŋ tə ðə ˈstɔr] (IEm gOH-ing tuh thuh stOR). This is discussed in the next section.

Any English letter that represents a vowel, ‘a,’ ‘e,’ ‘i,’ ‘o,’ or ‘u’ can be reduced to schwa [ə] (or another more central vowel) in an unstressed syllable. Occasionally (more often in some dialects than others) [i] is reduced to [ɪ]]. These reductions make English spelling difficult because you can’t sound out the spelling of the word.

What follows are lists of words of one, two, three, and four syllables in four formats. First is the word itself (a gloss), next is the world with the stressed syllable in capital letters, then the word in using a phonetic spelling with the stressed vowel in capital letters, and final the IPA (International Phonetic Alphabet) spelling. In the IPA spelling, the stressed syllable is proceeded by a stress marker (ˈ).

The lists have phonetic spelling and the IPA spelling of the way that the word is said. In most cases, one of the vowels changes. There are some syllables that are never stressed and even when the word is pronounced slowly you do not make the full vowel sounds.  For instance, the ending ‘-ed’ [əd] for the past tense is never stressed, but it can be stressed in “bed” [bɛd]

Be sure and notice that not every unstressed vowel is reduced to schwa (ə), in some cases, [i] is reduced to [ɪ] which lower the [i] and makes it more central (see the vowel chart).