The common definition of phonology is the study of sounds or phonemes of a language. Sounds or phonemes of a language are not necessarily the same as the other languages. The sounds of English, for examples, are different from sounds of Indonesian. The differences occur in every segment of phonemes, such as: Vowel, dipthong, consonant (segmental phonemes);stress, intonation and juncture (supra-segmental phonemes).
Reffering back to the topic that is English phonology, it means that we are concerned with the sounds or phonemes of English to initiate the explanation of English sounds or English phonemes, it is better to start first the diffrences between sounds and phonemes as well as phonemes and letters.
Phoneme is the smallest unit of sound, which can bring about the change of meaning. From the definition, we know that phonem must be sound, but sound may not a phoneme (except it can change meaning). Phoneme is limited in number, while sounds are unlimited in number, in English, for example: long, vowels because / I: :, U: :, כ: :, α: :/ are phonemic but in Indonesian they are not phonemic because they do not change the meaning of words. For proves, look at the following illustrations below:
English / bit / - / bi:t / ‘bit - beat’
/ bid / - / bi:d / ‘bid - bead’
/ k۸t / - / ka:t / ‘cut - card’
/ l۸k / - / la:k / ‘luck - lark’
Indonesian */ I:ni / - / InI / ‘this - this’
*/ ma:lu / - / malu / ‘ashame - ashame’
/ muk / - */ mu:k / ‘face - face’