Thank you.. Native speaks "slang" everyday?

I guess that speaking for to understanding only needs a little  few vocabularies, but we can speak fluently (using grammar). This one is proper for beginner, like me. But, sometimes we need to speak but only for style. The problem is that people have many vacabularies vocabularies, but they cannot be understood because their speaking isn't grammatical.
Do english native-speaker always speak grammatically?

 I think native- speaker doesn't speak grammatically. They usally use " slang".