"Are we writing a letter?"
This accepted „Czy piszemy list?” but I was surprised to see the canonical answer was shown as „Piszemy list?” When is it correct to leave out „czy” in a question like this?
„Czy” helps to disambiguate your question from a nomal sentence, but most of the time the intonation or the question mark works just fine.
I would encourage learners to use "czy" all the time. It always appropriate, you cannot be rude, and it is clear it is a question, which with beginners pronunciation could be a problem.
You just have to learn that usually you just add "czy" at the beginning, but sometimes in short sentences word order changes, so pronouns, "się", and "something" are not at the end.
Piszemy coś? Czy coś piszemy?
Perhaps you are right, but once you feel more comfortable, you should start dropping them to be more efficient. It should'n be that hard to learn for European language speakers to learn that intonation as it's not really that much different probably from the accent in their questions.
South Romance and other Slavic languages speakers should be familiar with this concept. Generally this is the only way for them to form questions in their native languages.
When I hover over "letter", it also shows litera. But when I put "Czy piszemy litera?" as the answer, I get it marked as incorrect. Am I missing something?
„Litera” is just a character, like A, B, C. The letter that you send to someone is „list'.