〜くなかった

1. Origins, structure, and basic meaning

• Structure: Change an i-adjective's negative くない form to the past negative くなかった.
  • 難しい → 難しくない → 難しくなかった
• Basic usage: It states that a quality or state did not hold in the past.

2. Common pitfalls and similar expressions

• Negating the expected degree
  • 思ったほど難しくなかった means it was not as difficult as expected. It does not strongly assert that there was no difficulty at all.
• Two polite forms
  • Both 難しくなかったです and 難しくありませんでした politely express the past negative. Do not combine them by marking the past twice in one form.
• いい and na-adjectives
  • いい becomes よくなかった, while a na-adjective takes a form such as 静かではなかった. Distinguish the connections by adjective class.