When learning another language from the ground up you learn the grammatical and pronunciation standards as a basis. Unless it is a language with very flexible grammatical patterns (like Tagalog), then there are usually set grammatical rules you learn from the get-go like word order, verb conjugations (if any), classifiers -- every language has different rules and limits.

For instance monosyllabic languages, such as all forms of Chinese and Vietnamese, form more complex words with combinations of one-syllable words. A simple example of this is the word beef: 牛肉 -- cow meat. While in English we have a whole separate word, beef, for cow meat.

Every language has its own complexities that native speakers initially learn from experience, whereas those who study to learn a language must study those same complexities indepth.

When someone, such as a natve speaker, is familiar with a language they get more comfortable with it and learn to "play" with how sentences and words are stuctured. Some scholars will say this is laziness, but most of the time it is for pure convenience in casual speaking. Even someone learning a language learns these "shortcuts" that people who speak the language use everyday. Sometimes it is just not the right situation to be precise with your speaking.

A very simple example of this in English is the already slang term "What's up?" becoming "'Sup?". It is all out of convenience and comfort in a language. These types of shortenings and slang happen in every part of the world, in every language.

Verbal communication is constantly evolving, with new dialects and ways to say things being constructed and used. A native speaker may not be speaking properly, but that is the beauty of language. It can be molded and formed to fit personal preferences and social instances.