The answer to this question is plain and simple: very, if not the most important thing. You see, English is a stressed language as opposed to others that are considered syllabic languages. It means that stress is what carries most of the meaning in spoken language, rather than syllables. There is a very practical way of getting to understand this. Get hold of any phonetic transcript, that is to say, check a phonetic book, where you may find a whole paragraph or even longer extensions transcribed into the International Phonetic Alphabet. If you could lay your hands on a tape with the oral version of the text in question, so much the better.

Though you might get bewildered at first, take each sentence at a time and try to analyze it. You will notice only few syllables carry actual vowel sound, the rest have that undefined sound called "Schwa". Here it is how it is most often represented:
Now, take a second look: it all makes sense. Only the stressed syllables carry a different, distinctive sound, all the rest reduces to schwa. At this point it is important to make a distinction between content words -stressed words that carry the actual meaning of a sentence, mostly nouns, adjective and verbs-, and function words. Function words are connectives, auxiliary verbs, prepositions, conjunction, pronouns, mostly single-syllabic words. These words, function words, never, and this cannot be stressed enough, never carry any other vowel sound than our friend, the Schwa sound.

A bit too radical for you? Not really, just give it a try. Now focus your attention to the consonants, and take a closer look. Try to differentiate the different consonant sounds and variations. Is it the same /b/ when it is at the beginning of a sentence or stressed word, between voiced sounds, ie.: between vowels, or at the end of sentence or stressed word?

Now, a little game. Take any of the sentences of the text in question. Now read it out loud, but instead of using traditional vowels, use the Schwa sound every time. If you can provide the correct pattern of intonation and accentuation, I can assure to you, it will sound really close to native American English.

We have to conclude that excessive vowel differentiation is one of the most noticeable foreign marks you may encounter in oral English.

Another mini game, try these two sentences:
a- I can do it
b- I can't do it
Pretty much the same, right? Only a letter and apostrophe of difference. Nevertheless, their intonation is radically different. "I can do it" carries the main stress on "do", whereas "I can't do it" carries the main stress weight on "can't". Really illustrative, don't you think so?

Examples like this multiply in every day conversation. Meaning is primarily carried by intonation and not vowel differentiation. Give it a try. Though at first it might sound to you as a little exaggerated, once you pay close attention to colloquial English -not declamatory English or poetry, for instance, just people talking, you will notice most sounds are actually the Schwa sound. Master this, and the door will be opened to let you into a more advanced stage in your domain of the English language.

If you enjoyed this article, please feel free to post it to your site or blog and forward this link to your friends. Have a great day!"

Don't forget to visit my ESL blog http://www.englishlci.com/esl-school-1.html.