I'll give an example:
A native speaker will generally stress content words (for example: nouns, verbs, adjectives, adverbs). However, it is very uncommon for a native to stress a function word (auxiliary verbs, articles, demonstratives). Consider this sentence:
Jack could have been taking the train to London.
A native speaker will stress only the content words, like so:
Jack could have been taking the train to London.
The other words are function words (even the -ing in taking), so they are pronounced with a weak vowel (called the schwa). This weak vowel is not strong, and sounds very short and unclear. Furthermore, these function words will "change" there pronunciation in fast speech. It will sound more like this:
Jack kədəvbən takən thə train tə London.
So "could have been" kind of becomes one word blended together, "kədəvbən." Step by step:
1. could --> /kəd/
2. have --> /əv/ (the h sound is deleted)
3. been --> /bən/
Some speakers might even speak faster and pronounce have as just a vowel /ə/. They might also change the /t/ in to so it assimilates to the sound before it (the /n/ in train) For example:
Jack kədəbən takən thə train nə London.
So if you want to sound foreign, pronounce every vowel with full strength and volume, but if you want to sound more native-like, put stress on the content words and then make the other words in the sentence weaker.
Just take some time to listen to native speakers talking and try to pay attention to how they pronounce content words and function words. You will hear a difference, I promise.
Have a good one, and good luck out there!
-Bill