確か

1. Origins, structure, and basic meaning

• Structure: 確か (たしか) can assert certainty or cautiously introduce a recollection that may not be complete.
• Basic usage: One speaker confirms a visit as fact; another relies on memory when recalling where a key was placed.

2. Common pitfalls and similar expressions

• Predicative certainty and sentence-initial recollection
  • 確かだ judges a proposition as certain, while sentence-initial 確か is closer to “if I remember correctly” and leaves some reservation. They do not express identical confidence.