Quote Originally Posted by schildt
What about the sentence building: Is there any way to explain why I shouldn't put писать по русский at the beginning of the sentence?
You can actually say "Писать по-русски мне трудно" too, but the other variant seems more natural to me in this context. Often there are no strict rules for the sentence building. It often depends on the context, intonation, words you want to emphasize etc. With experience it will become easier.