Is this just a bad example sentence? 'Jablko to czerwone' seems like it should be perfectly reasonable but is being marked wrong
You forgot about the letter "ł" which is similar to "l". The correct answer is: "Jabłko jest czerwone".
My question was more about whether 'to' could replace 'jest' in this sentence. In the exercise, I used the correct spelling of 'jablko'
No, you can't use "to" with adjectives because it can be compared to "=" so it has narrower application than the verb "być" (to be).
why is czerwony not in its instrumental case here? i thought any word following jest, jestem, jestes, jestescie was instrumental but im most likely wrong :D
You took the rule too far and it's not a surprise :) A noun phrase following a form of "być" takes Instrumental. But if it's just an adjective, it stays in Nominative.
Why "czerwone" and not "czerwony"?
"czerwone" is neuter singular, "czerwony" is masculine singular.
"jabłko" is neuter.