"My son is watching the deer."
Singular or plural? Yes, in general English is more consistent with its pluralization of words (usually just -(e)s ), but every language has its own fun with plurality. And this example in English doesn't have the nicety of das/die. And as painful as it may seem, the better you get at it, the more natural you will sounds. And that to me is an amazing feeling.
I find it particularly difficult to remember a new word if, at first presentation, it is given as a plural. I'm always thinking of it as "die" like this and I need to think of the new word and the correct gender. This seems particularly important in German. I'm reporting it as a problem - perhaps it may be improved one day.....?
It works the same in German as it does in English. It you use 'the' (or 'die' in German) you are referring to a particular group of mirrors. Without it, you are talking about mirrors in general. Since the article appears in the German sentence you need to have it in the English.