下

1. Origins, structure, and basic meaning

• Structure: 下 is read した here and denotes a position below a reference object.
• Basic usage: In 机の下に落ちた鉛筆を拾った。, the pencil was picked up from beneath the desk.

2. Common pitfalls and similar expressions

• A space and a route through it
  • 机の下 names the space beneath the desk, while 橋の下を marks the route taken by the boat. The examples distinguish a position below something from movement through that space.