〜かった

1. Origins, structure, and basic meaning

• Structure: Change the final い of an i-adjective to かった.
  • 難しい → 難しかった, 冷たい → 冷たかった
• Basic usage: It describes a quality or state in the past.
  • 昨日の試験は難しかったです。 states that yesterday's exam was difficult.

2. Common pitfalls and similar expressions

• Polite past forms
  • In 難しかったです, the adjective already carries past tense and です adds politeness. This differs from where the past is formed in the na-adjective expression 静かでした.
• Change in いい
  • The past of いい is よかった. Learn it with the forms based on よい that appear in inflection.
• Relation to the current state
  • この公園は広かった gives a past assessment. The surrounding context tells us whether the park is still spacious or has changed.