You can use an extended vocabulary only by entering into a discussion of ideas. Discussing Abstract Conceptions will permit the articulation of an extended vocabulary. So long as dialogue is limited to material objects, a simple vocabulary will suffice.

The best way to speak English well is to practice, practice, practice: listening to it, speaking it, and reading it. I recommend that my students listen to English TV and radio, and read English newspapers and magazines. Even though you will not understand everything at first, you will eventually understand more and more words. Since you already have a good English vocabulary, it might be a good idea to find a teacher who specializes in conversation, so that you can focus on putting that vocabulary to good use! Good luck!

Hello buddy. just find a teacher or partner and speak more and more. It's the best method for this purpose and you can speak more fluently soon.