I go to school everyday. [错!Everyday是形容词,go to school是动词词组。形容词不可以修饰动词。] I go to school every day. [对! Every day是副词词组。副词可以修饰动词。]
还有,everyday虽然是形容词,但它只作定语,不作表语。所以,你可以说an everyday situation/issue,但你不可以说The situation/issue is everyday.(大多数形容词是既可以作定语,又可以作表语的,比如beautiful flowers/the flowers are beautiful。)