Phonetics / Phonology Phonetics has been defined by Roach
Phonetics has been defined by Roach (1992) as the 'scientific study  of speech' (Roach, 1992: 81.) It is concerned with how speech sounds  are formed, how we use them in our spoken language and how we can  record these speech sounds using written symbols. Phonology on the  other hand has been defined by Roach (1992) as 'the study of the  sound systems of languages' (Roach, 1992: 82.)  
According to Roca and Johnson (1999), the way in which we speak is  somewhat similar to playing a recorder. This is because when you  play a recorder you have to blow air from your lungs, as you do when  you speak. However, to play a tune, you cannot simply blow air into  the recorder; you must have your fingers over specific holes.  Therefore, when we speak, we have to interfere with the air that  comes out of our mouths to produce a variety of different sounds.  Roca and Johnson (1999) gave the example where you 'place your lower  lip loosely on the lower edge of your upper teeth and force the air  out.' (Roca, 1999: 6) The sound that you produce here should sound  like the letter 'f'. However, a problem can occur when attempting to  spell words, particularly for those learning English. For example,  the 'f' in the word food and the 'gh' in the word cough are  pronounced identically yet spelt very differently. This suggests  that the English spelling system is not a completely accurate way of  describing how we pronounce sounds. As a result, The International  Phonetic Association identified a transcription system where one  symbol stands for a specific sound. Consequently, if foreign  students learn this alphabet, they should be able to correctly  pronounce any word in English. 
Another important area of pronunciation that students should study  is that of stress and intonation. Harmer (1998) explains how stress  is related to emphasis that is placed on specific words or  sentences. Therefore, when we pronounce the syllable with the added  stress; our voice will either increase in volume or change pitch.  Roach (1992) explains that 'the position of stress can change the  meaning of a word' (Roach, 1992: 102.) or sentence. For example, if  the stress in the sentence 'he played football yesterday' was on the  word 'he,' we would imagine that it was him that played football.  However, if the stress was on the word 'played', we would imagine  that he only played football, not anything else. Furthermore, we  frequently vary the pitch of our voice in a sentence. Roach (1992)  believes that this conveys our emotions and attitudes; an example of  this is when we are excited as here it is likely we will have a  higher pitch of voice yet we will have a lower voice when we are  bored. Often when we speak, we use a rise/fall intonation. This is  normally perceived as a polite way of speaking and inviting the next  person to speak. However, we also use a fall/rise pattern which  indicates an element of surprise or disagreement with something that  has been said by a previous speaker. It is therefore important that  stress and intonation are studied by English students as this will  help them understand the meaning of others sentences.
It can therefore be seen from the above information that it is very  important to teach students learning English phonology and  phonetics. This is because they are essential when trying to  understand the meaning of people's sentences and when trying to  learn how to pronounce certain words.  
References
Harmer, J. (1998). How to Teach English. England: Longman.
Roach, P. (1992). Introducing Phonetics. London: Penguin Books. 
Roca, I. and Johnson, W. (1999). A course in Phonology. Oxford:  Blackwell Publishing.   




