Native speakers use reduced forms at all levels of English. Some forms are more common in faster speech, others are used at any speed. It is something that is learned in childhood by listening and imitating, not at school. It is part of the natural rhythm of the English language, whether you speak with a British, American, or Australian accent, even though most native speakers are not aware that they regularly use reduced forms. If you don't make an effort to learn the theory behind reduced forms, it will be difficult for you to understand native speakers when they talk with each other.
English is a stress-timed language. It has regularly spaced accents or stressed syllables, whereas many other languages. like Russian, have equally spaced syllables and are syllable-timed. The use of reduced forms is directly related to the rise and fall of stress in an English sentence.
Don't be discouraged. Learning about the rhythm of the English language will make it easier for you to speak fluenty, so it is well worth the effort.