“相反”的英语表达是 “on the contrary” 或 “to the contrary.” 这两个词都表示相反的意思，但在句子中的用法略有不同。

“On the contrary” 是一个固定短语，通常用于句子开头，表示对前面所说的话的否定。例如：

I don't think I'll be able to come to the party tonight. On the contrary, I'm looking forward to it.

Some people say that studying is a waste of time. On the contrary, I believe it's essential for success.

“To the contrary” 也表示相反的意思，但它可以用于句子的任何位置，而不仅仅是句首。例如：

I'm not tired at all. To the contrary, I feel wide awake.

You might think that I'm angry, but to the contrary, I'm just very disappointed.

两个词都来自拉丁语，“on the contrary” 来自 “contrarius”，意为“对立的”或“相反的”，而 “to the contrary” 来自 “contra”，意为“反对”或“相反”。