Breakdown of Adres piszemy na kopercie.
na
on, at / to (motion toward) / for (a meal) / for (waiting for)
pisać
to write
adres
address
koperta
envelope
Questions & Answers about Adres piszemy na kopercie.
Is adres the subject of this sentence since it comes first?
No, the implied subject is "we", which is built into the verb piszemy (we write). Adres is the direct object (the thing being written). Polish word order is flexible, so putting the object first just makes it the main topic of the sentence.
If adres is the direct object, shouldn't it change its ending for the accusative case?
It is actually in the accusative case right now! Because adres is a non-living masculine noun, its accusative form is exactly the same as its dictionary (nominative) form.
Why does the word for envelope (koperta) change to kopercie here?
We are expressing a static location ("on the envelope"), so the preposition na forces the noun into the locative case. For feminine nouns ending in -ta, the ending softens to -cie in this case.
Would the meaning change if I used the preposition w instead of na?
Yes. Just like in English, na kopercie means writing on the outer surface of the envelope. If you used w kopercie, it would mean you are writing something inside the envelope.