"Your generation is not good."
Translation:Wasze pokolenie nie jest dobre.
"Pokolenie" is a narrower term. It refers strictly to generations of people. Using "generacja" in this sentence might be technically correct, but too fancy-sounding.
"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.