"I eat fast" 英文表达方式为 "I eat quickly"。

"quickly" 是一个副词，意思是 "快地"。它通常用来修饰动词，表示动作的速度。

举例：

I eat quickly. (我吃饭很快。)
She runs quickly. (她跑步很快。)
He speaks quickly. (他说话很快。)

还有一种可能的说法是 "I eat at a fast pace." , fast pace 意思是快速步伐。
举例：

I eat at a fast pace. (我吃饭速度很快。)
He walks at a fast pace. (他走路速度很快。)