"Your generation is not good."
Translation:Wasze pokolenie nie jest dobre.
"nie" is written together with adjectives in their positive (basic) form. So "niedobre".
But it doesn't exactly work this way. Often after adding "nie", the adjective changes its meaning. For example, "bezpieczny" is "safe", but "niebezpieczny" doesn't mean "not safe", it means "dangerous". So "On nie jest bezpieczny" and "On jest niebezpieczny" are totally different sentences.
With "niedobre", I think 9/10 people would interpret it as "not good" meaning "not tasty".
You might have heard about Polish using so-called double negation (or even triple sometimes). This applies to words for "nowhere", "nobody", "never", etc; we use them in negative sentences, even if the verb was negated already. But with adjectives it's more… normal. You can negate the verb or the adjective, but not both, because they will kinda cancel each other out.
If both X and Y are nouns, you can use either version, but there is a catch. If you use the version with "to", then Y is in nominative case (the most basic one). If you us the other, then Y needs to be in instrumental case.
If Y is an adjective, you can't use "to", but Y stays in nominative regardless.