"Walking is boring."
Translation:Chodzenie jest nudne.
In an "X is Y" sentence, you can only translate it using the construction with 'to' if Y is a noun phrase.
So for example "Chodzenie to nudna czynność" (Walking is a boring activity) is correct.
But "Chodzenie to nudne" is wrong, because after 'to' you only have an adjective.
See here: https://www.duolingo.com/comment/1637316 - especially Part 3. The gerund "chodzenie" is still just a noun, so it doesn't complicate anything further.