に

1. Origins, structure, and basic meaning

• Structure: Add に after the place where someone or something exists or stays.
• Basic usage: 机の引き出しに鍵があります。 gives the key's location.

2. Common pitfalls and similar expressions

• Place and existing thing
  • In 駅の前に時計がある, に marks the location and が the thing present. The location still takes に when the clock becomes the topic, as in 時計は駅の前にある.
• Describing an activity
  • 部屋にいる describes being in a room, while 部屋で仕事をする describes an activity there. Choose according to the Japanese predicate rather than a one-to-one translation of a location marker.