A conjunction/particle that expresses contrast or an unexpected result (although, despite, even though).
雨が降っているのに、彼は傘を持って行かなかった。
Even though it was raining, he didn't take an umbrella.
時間がないのに、長話をしてしまった。
Although there was no time, we ended up having a long conversation.
医者にタバコをやめるよう言われたのに、まだ吸っている。
Even though the doctor told him to quit smoking, he still smokes.
お金がないのに、あの店で高い服を買った。
Even though she had no money, she bought expensive clothes at that store.
Used to contrast two facts or states (similar to 'whereas' or 'while'); often overlaps with the 'although' usage.
姉は社交的なのに、弟は静かだ。
Whereas the older sister is outgoing, the younger brother is quiet.
東京は暑いのに、北海道は涼しい。
While Tokyo is hot, Hokkaido is cool.
彼は料理が得意なのに、掃除はあまりしない。
He is good at cooking, whereas he doesn't do much cleaning.
Expresses regret or wishful thinking (if only, I wish), often in patterns like '〜たらいいのに' or '〜てくれればいいのに'.
もっと時間があればいいのに。
I wish I had more time.
彼が来てくれたらいいのに。
I wish he would come.
もっと早く教えてくれればよかったのに。
I wish you had told me earlier.
A nominalizing use (verb + のに) meaning 'in order to' or 'for (the purpose of)', as in 'to do X (it is necessary/used)'.
日本語を話すのに毎日練習が必要だ。
To speak Japanese, daily practice is necessary.
この鍵はドアを開けるのに使う。
This key is used to open the door.
料理するのに時間がかかる。
It takes time to cook.