"Ne, ty hrady velké nejsou."

[EricAllard2]

Why not, "No, those are not big castles."?

[endless_sleeper]

The Golden Rule of Duolingo: Translate as closely as possible.

'No, those castles are not big.' = Ne, ty hrady nejsou velké/velké nejsou.

'No, those are not big castles.' = Ne, to nejsou velké hrady.

In another way, in the first sentence, the adjective is connected to the verb, in the second sentence, to the noun.

[DougExner]

I disagree. In the German lessons it is not that way at all. The English doesn't generally come out awkward and strange like this accepted answer. I'll report it.

[FutureOfDenmark]

Why nejsou in the end

[endless_sleeper]

Additional emphasis. We usually put the most important part at the end of the sentence.

[Enya_Small]

Is nejsou a negative?

[goldregenb]

what is the difference between "these" (failed), and those?

[VladaFu]

There are many explanations on the internet. Just search and find stuff like https://learnenglish.britishcouncil.org/en/english-grammar/that-these-and-those