看到尼亚加拉瀑布是一次难忘的经历。

Seeing Niagara Falls was an ___unforgettable experience.

我刚到家，想感谢你让我度过了一个难忘的周末。

I've just arrived home and I'd like to thank you for the ___unforgettable weekend.

那场音乐会绝对令人难忘。

The concert was absolutely ___unforgettable.