Never forget this lesson.

永远不要忘记这个教训。

《新英汉大辞典》

Ralph would never forget this.

拉尔夫永远不会忘记这件事。

youdao

I'll never forget this exciting summer vacation.

我永远也忘不了这个激动人心的暑假。

youdao

This is the worst thing that has ever happened to me and I will never forget this so long as I live.

BBC: The survivors' stories

"Never forget this: The Lord never tires of forgiving us, " he said.

CNN: March 17, 2013 -- Updated 1658 GMT (0058 HKT)

And that I too would never forget this day.

BBC: Piecing together puzzles in Cambodia