When producing the word banana ,one can identify three identifiable syllables.The second syllable in this word sounds louder than the other syllables,so one gets ba'nana.This stressed syllable also has a change in the pitch or the level of the speaker's voice,and the vowel sound in that syllable is lengthened.Thus for stressed syllables three features are identified : loudness , pitch and length
2) What is Unstress :
In order for one syllable to be perceived as stressed ,the syllable around it need to be unstressed.In the word syllabus for instance the first syllable is stressed ,this implies that the finale two syllables are unstressed .one my know that the most commonly occurring vowel sound in English known as "schwa" never appears within a stressed syllable.schwa is by nature an unstressed syllable.If one tries to stress any syllable which contains schwa,he will change its properties producing another phoneme.
3) The importance of sentence stress :
Any one who has listened to any native speaker of English will be aware that some words are given more prominence than others in the same sentence.For example "Her brother bought her a new coat for Christmas" in this sentence a native speaker would stress:brother,bought,new,coat,Christmas.one would not stress : her,a,for.In this way ,it is possible to divide the sentence into strong syllables ans weak syllables.This distinction is important since native speakers swallows the weak syllables.
Content words and form words :
consider the following sentence : " The student wants a drink of water".If this water had to be obtained by telegram,and if one had to pay for each word ,he would be unwilling to send the whole of this message and would probably send "Student wants drink water".These four words convey the whole message of the sentence and are for this reason known as Content words or Lexical words.These content words are always stressed and they include :NOUNS,VERBS,ADJECTIVES,and ADVERBS.The other words in the sentence exist for grammatical convention ,and do not change or add to the meaning of the sentence.These are called Form words or Grammatical words.They exist to satisfy the grammatical form of the sentence.They comprise :ARTICLES ,PREPOSITIONS,POSSESSIVE ADJECTIVES ,PERSONAL PRONOUNS,AUXILIARIES ,CONJUNCTIONS , and RELATIVES PRONOUNS.All form words are usually unstressed but there are exceptions,some form words tend to be stressed as :The demonstrative pronouns,possessive pronouns,interrogative pronouns,post position (after the word),interjections.Since "not" maybe considered as a kind of adverbs or pronouns as:nobody,no one,nothing,....are always stressed