Many native speakers of English speak in a way which makes them very difficult to understand. They may have different regional accents, or speak with the accent of their country. Also, as with all languages, native speakers tend to leave out some of the words as they speak, which is not a problem for other native speakers. I suggest you listen to the BBC World Service in English on the radio [or its equivalent in other countries], as presenters speak slowly and clearly for an audience of non-native listeners.