He drove so carelessly that his car crashed into a tree.为什么不用careless?

careless 是形容词，carelessly是副词。动词drive后面应该跟副词

carelessly 是副词，他可以修饰动词，如此句中的drove,但careless是形容词，一般修饰名词，如he is a careless boy本回答被提问者采纳

carelessly是副词，可以修饰动词；careless是形容词，不能修饰动词！