• Wow, I wish I could write like this. Can you give me some tips? 
• This is a really good answer, I can’t see any mistakes.
• Great vocabulary, I didn’t know the word _____ and ______.
• Very clever ideas. They clearly relate to the question. 

Good

• Your grammar is very good. However, I think you made a mistake in the sentence _______. I think it should be _______.
• Your introduction/overview is really good. However, I think ______ is a mistake. It should be _________.
• In general, your spelling is good but you misspelled the word ______.