Knowing when to add stress in a sentence can be a little difficult, and some words can change their sound if they are unstressed. In English we stress the content words in a sentence, the ones that convey meaning to the listener. Generally this means that nouns, adjectives, verbs, adverbs and negation words are all stressed in a sentence. Smaller words that carry no content, like prepositions and articles, are usually unstressed. Not only are unstressed words generally said quieter, but their vowel sounds are replaced with a schwa.
“I am going to the store to buy some coffee” would be strange to hear, as if every single word in the sentence is very important for the listener.
“I’m going to the store to buy some coffee” is more natural.
to (tu) becomes to (tə), the (ði) becomes the (ðə), some (sʌm) becomes some (səm)
All of the vowels have been replaced with the schwa (ə), the sound that the human moth makes when the throat, lips and tongue are relaxed and you just let a sound come out. (ə)
The bad news is that this means that half the words in a sentence you hear can become unintelligible, said quietly and with altered pronunciation. The good news is that you can ignore them and still understand the sentence.
“I’m going to the store to buy some coffee”
“I’m going store buy coffee” tells you everything you need to know.
Of course you can choose to add stress to non-content words if they are important. For example instead of saying this is “the end” I could say this is “the end” emphasizing the finality.