"感到吃惊"的英语表达可以用"be surprised"或"be astonished"。

"Be surprised"是"感到惊讶"的常用表达方式，它来自英语动词"surprise"，意思是"使惊讶"。例如：

I was surprised to see him at the party. (我很吃惊地看到他出现在聚会上。)

"Be astonished"是"感到惊讶"的更强烈的表达方式，它来自英语动词"astonish"，意思是"使惊讶"。例如：

I was astonished by the news of her sudden death. (我听到她突然去世的消息很吃惊。)

此外，还可以使用一些其他表示"感到吃惊"的短语，例如"be shocked"（感到震惊）、"be amazed"（感到惊奇）、"be flabbergasted"（感到惊愕）等。例如：

I was shocked to hear about the accident. (我听到这起事故很震惊。)
She was amazed by the beauty of the city. (她对这座城市的美丽感到惊奇。)
He was flabbergasted by the news. (他听到这个消息感到惊愕。)