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Mark Davies & Dee Gardner. a FREQUENCY dictionary of CONTEMPORARY AMERICAN ENGLISH en el Prefacio:
"60 percent of speech in English is comprised of a mere 50 function words"
60 percent of spoken English is composed of 50 function words. The bad news is that they reduce and link them as if forming new words.
Both vowels and consonants are pronounced with less emphasis. Try to observe it mainly in the videos of Youglish.com, since it is not that they can not pronounce them correctly, but that their language requires them to reduce them. And since you have decided to learn this language, you will have to do it too. Look in the pronunciation field for multiple pronunciations, in some words.
The list includes the words Function within the 5000 words of English language. Once the word is selected in the list, you can click on the word in the link to the right, to see videos from Remember the facilities that it offers.