Есть у меня какие нибудь ошибки тут? this date will never erased from history, she will remain forever in the calendar. the heroism of the fallen soldiers immortal. and we must remember what the cost of our happiness.

This date will never be erased from history, it will remain forever in the calendar. 
The heroism of the fallen soldiers is immortal.
And we must remember the cost of our happiness. (лучше без "what")