Only working hard and improving more is the key to success.

working hard and improving more 是动名词作主语.work hard ,improve 是动词,不能做主语.only 是副词,作状语,强调主语.副词只能作状语.定语需要形容词、相当于形容词的短语（形容词短语）或者相当于形容词的句子（定语从句）.你的语法很糊涂,建议用薄冰语法.