古代

1. Origins, structure, and basic meaning

• Structure: 古代（こだい） refers to ancient times as a historical period.
• Basic usage: 古代の道具 means tools from that period; 古代を舞台にしている means a story is set in antiquity.

2. Common pitfalls and similar expressions

• The past in general and a historical period
  • 昔 broadly refers to the past, while 古代 is used as a historical period designation. Its precise dates depend on the region and historical context.