Things I've learned in this class

The most impressive thing I learned from this class is usage of "but" and "however".
I was very surprised that "but" shouldn't put first of the sentence because I rarely use "however" and always use "but". It means I had been made mistakes all the time when I write English since "but" is my favorite word when I write English. It appears my English very often.
Big problem for my English was solved for this class.
I really appriciate for it.