Written and spoken language are fundamentally different. They use different organs to receive (eyes and ears) and are processed in the brain differently. Because of this, they use different sentence structures and follow different grammatical rules.
Despite both being contemporary English, writing and speech are different languages. If you try to use one to do the other, it will probably come off poorly.
Learn the differences between writing and speaking. Sometimes the best way to do it is by watching speeches and reading articles. You can absorb a lot of style by just surrounding yourself with it. Once you've done that, you will readily notice when somebody does it wrong.