'Much' here is a noun. It is the direct object of 'learn'.

作名词。 窍门：如果是名词，那就可以用 a lot 来替换。替换完以后如果意思和语法都说得通那就没错了。 区别： I don't have much money. 这里就是形容词了。那就不能是 a lot，就得是 a lot of才行