のに

Even though
Plain form verb/い Adjective + のに
な Adjective/Noun ＋なのに

のに is a particle that means “even though”. It is usually used for complaints or criticism. 
It usually comes across as judgmental when used about other people; and whiny when used about yourself. Nuance wise, it seems closer to “Despite the fact that..”

What comes after のに should be contrary to expectations.
Emphasis is placed on the word before のに. 
Nouns must use な like な adjectives.

Examples

• 大人なのに泣くなんて – (I can’t believe) they’re crying even though they’re an adult. 
• 僕は頑張ったのに勝てなかった – I lost even though I tried hard. (whine)
• 彼は頑張ったのに勝てなかった – Even though he did his best he lost (Sympathetic).
• 若いのにもう疲れている？ – You’re already tired even though you’re so young?
• 子供なのに殴った？ – You hit them even though they are a kid? 
• 暇なのに手伝ってくれないの？ – You won’t help me even though you’re free?
• 新しいのにすぐ壊れた – It broke (quickly) even though it was new. 

Non-Complaints (Rare)

Although usually complaint-like, this is not always the case:

• 土曜日なのに学校に行ってしまった – Even though it was Saturday, I accidentally went to school. 
• 彼はまだ子供なのに自分の意見をしっかりと持っていてすごいと思った。(from weblio)
  Despite the fact that he’s still a child, I think its amazing he has his own firm opinions.