I opine that "opine" is not that commonly in use. Personally, I don't feel using fancy words improves the quality of a language, unless, maybe, sometimes, if your English is already at a very high level, grammatically nearly perfect, and if you know really well in which situation and context you can use those words. Often, if used in an informal situation or in a wrong way, those fancy words may look strange and inappropriate, and might make your English sound funny rather than increase its level. A good way to pick up words is to read and listen a lot. Then they will come naturally. For example, pick a couple of native English speakers whose writing you like, writing in a context that you need to write in, and analyse their writing. Learn the words and expressions that they use. So these are your new words, fancy or not.

Hi Katarina,
Thanks for fast answer! Surely I understand that I can't just replace some word with a "fancy" word :)
I just want to find out which of them are commonly in use, and then I'll google to find out how to use them correctly. I opine it'll increase my level, I mean the quality of the language.