fast是副词,副词修饰形容词和动词,但这句是修饰动词,所以不能用be
这里有一些例句,你能看以下哦!
主语+谓语(系动词)+as+形容词原形+as+从句.表示两者对比相同.
This box is as big as mine.
这个盒子和我的一样大.
This coat is as cheap as that one.
这件衣服同那件衣服一样便宜.
I study English as hard as my brother.
我同我兄弟一样学习努力.