"по-X " is answering on the question "How? How it is?". This is an adverb. Examples: "говорить по-английСКИ" _BUT_ "по английскому языку" "не могу по-другОМУ" _BUT_ "по другому поводу" To sum up, the preposition "по"+ the end of the word "СКИ" or "ОМУ" needs a dash between preposition and adjective. Togetger they works as an adverb. But if you have a word that refers to adjective, it takes away the dash.(по другому поводу, учебник по русскому языку)