一つ

1. Origins, structure, and basic meaning

• Structure: 一つ(ひとつ) is used to count one object or item.
• Basic usage: It describes choosing one red apple from a basket.
  • 籠から赤いりんごを一つ選んだ。

2. Common pitfalls and similar expressions

• Objects and abstract items
  • りんごを一つ counts one apple, while 質問を一つ counts one question.
• One item and one occasion
  • 一つ counts items, while 一回(いっかい) counts occasions. 質問を一つする means asking one question; 一回質問する focuses on performing the act of asking once.