固める

1. Origins, structure, and basic meaning

• Structure: 固める (かためる) is a transitive verb meaning to harden a soft or liquid material or to settle an undecided plan.
• Basic usage: Its object can be a physical substance or an abstract policy or decision.
  • 砂糖を加えて冷やし、果汁を固めた。 — We added sugar and cooled the fruit juice until it set.
  • 話し合いを重ねて今後の方針を固める。 — Through repeated discussions, we settle our policy for the future.

2. Common pitfalls and similar expressions

• Become firm and make firm
  • 固まる is intransitive for something setting or becoming decided, whereas 固める is transitive for making it firm or settled.