Berkay Gürgen https://validator.w3.org/feed/docs/rss2.html stem diagnostic test placement test proficiency level CEFR rubric band descriptor criterion accuracy fluency complexity range fossilized error self-correction peer feedback input output opinion essay discussion essay advantage disadvantage essay problem solution essay two-part question to what extent balanced opinion counterargument refutation concession elaboration unity redundancy wordiness conciseness clarity complex sentence compound sentence dependent clause independent clause subordinating conjunction coordinating conjunction parallel structure inversion reduced relative clause participle clause nominalization hedging generalization overgeneralization bar chart line graph pie chart