N5
の（疑問）
の（ぎもん）
Contextual and Explanatory Questions
私のお母さんとさつきちゃんのお母さんは友達なんですか？
Are my mom and Satsuki's mom friends?
Explanation
「の」 at the end of a sentence with rising intonation is the casual form of 「〜んですか」, used with family, close friends and children: 「どうしたの？」 (What's wrong?). Use it when you see something and want to ask why or check, in a warm, gentle tone.
Mixed up with
After a noun or a な-adjective use 「な」: 「休みなの？」, not 「休みだの？」.
Adding 「か」 straight after a casual sentence (「行くか？」) sounds harsh, like an interrogation; 「行くの？」 is much gentler and shows concern.
Men often just raise the voice (「行く？」) or say 「行くのか？」; women and children use 「〜の？」 more.