Finally, I get how it works, but it was not intuitive at all! The dev should make this extension more user friendly, with more explanations on the addon description page! Explanations are here: http://sifnos.sfs.uni-tuebingen.de/VIEW/index.jsp?content=intro

It's good to practise phrasal verbs, articles, and some other stuff, in English, Russian, German, Spanish. It would be cool to add more things, levels, type of exercises, audio or phonetics, gamification, more languages. It's cool, but it's very basic, too few kind of exercises. I still prefer Readlang for now.

Thanks for goog add-on. It has some bug about displaying font awesome. The add-on block showing font-awesom icons.