感叹句，只有what和how两种开头。看句子意思是，蛋糕发出了何等的香味！宾语是smell，tasty是修饰smell的，也就是说感叹的重点是名词smell，味道。所以这里要用what，后面接名词smell。叙述语序就是：the cake gave off tasty smell。如果要用how的话感叹句的重点应该是形容词tasty，句子应该是how tasty the smell that the cake gave off is！叙述语序就是：the smell that the cake gave off is tasty。

要区分这两种，就先把句子换成陈述语句，然后看感叹的那个重点变疑问句可以替换成what还是how。The cake gave off what。the smell is how。这么解释懂么。