What is the difference between "fabrika" and "továrna"?
Fabrika is only the colloquial espression. In written czech use Továrna.
Továrna is the neutral word.
Fabrika sounds more expressive, more colloquial. And it's obviously of German origin.
What's wrong with 'Před padesáti léty naše továrna byla otevřená'?
otevřená - open
otevřena - opened
And byla wants to be in the second position, if not first or final, but that is unlikely here.