"了不起"的英文是"impressive"。"Impressive"的来源是英文动词"impress"的形容词，意思是给人深刻印象的，令人印象深刻的。下面是一些例句：

That's an impressive accomplishment.

The view from the top of the mountain is truly impressive.

She gave an impressive performance in the play.

翻译：

那是一项了不起的成就。

从山顶看下去真的很了不起。

她在剧中的表演令人印象深刻。