Yes, drzwi in Polish is plurale tantum, like scissors or glasses in English. If you really want to stress that you want to say about just one door, you would say jedne drzwi (one (of the) doors).
EDIT: As pointed below, it would rather be jedne drzwi than para drzwi as I wrote before, now corrected.
You would say 'jedne drzwi'. According to the following website:
the phrase 'para drzwi' is incorrect (unless it's literally a pair of doors).
You are right. I have no idea why (or how) I created that pair of doors… My bad.
Why not "Otwieram drzwi z moim kluczem"? I thought you needed "z + instrumental" to make "with"? Or can the "z" be omitted?
It not only can, but have to be omitted here – „z” + instrumental is not viable translation of all meanings of with:
In the company of; alongside, along side of; close to; near to.
In addition to; as an accessory to.
Used to indicate simultaneous happening, or immediate succession or consequence.
In support of.
(obsolete) To denote the accomplishment of cause, means, instrument, etc; – sometimes equivalent to by.
Using as an instrument; by means of.
(obsolete) Using as nourishment; more recently replaced by on.
From these, you can use „z” + instrumental for meanings: 1, 2, 3, 5, 9(this one I base on the quote on wiktionary, which I would translate to Polish using z). Meaning 7 uses Instrumental case instead(that's why the case is called so, really ;) ).
As a native speaker, I interpret „Otwieram drzwi z moim kluczem” as using meaning 2, so it's sort of like 'me and my buddy the key are opening the door(s) together'… I would need to be really wasted to personify a key like that. ;)
Oh right! So if you're using meaning 7, you can just omit the "z" and just use the instrumental case? Thanks!! :D
"I open doors with my key." was counted as incorrect. Is that right? I thought drzwi was both plural and singular.
That's an oversight, "the doors" were already accepted. Added now. Indeed we do not know in this sentence whether it's singular or plural.
I often use the plural "keys" in a situation like this, even though I mean just one key, but it isn't accepted. Is this an intentional decision because the Polish uses singular, or is it just an oversight?
I think we should stick to a singular key if there was only one in the Polish sentence.
My confusion was with the missing z. I was expecting "Otwieram drzwi z moim kluczem"