確実 and 確か: Reliable Delivery and Recalled Certainty
Read checking a destination to ensure delivery and withholding certainty about success. Compare certainty that someone came with a recollection about leaving a key.
確実 (かくじつ): A delivery goal and a judgment
「忘れ物を確実に届けるため宛先を確認した。」 — “I checked the destination to ensure the lost item would be delivered.” The destination was checked to ensure delivery of the lost item. The completed check and its purpose are given; completed delivery is not.
「成功が確実だとはまだ言い切れない。」 — “We cannot yet say that success is certain.” Success cannot yet be called certain. This is not a statement that success is impossible or failure has already occurred.
確か (たしか): An assertion and a recollection
「彼がここに来たのは確かだ。」 — “It is certain that he came here.” The assertion is that he certainly came here. That arrival is the content about which certainty is expressed.
「確か、鍵は机の上に置いたはずだ。」 — “I am sure I left the key on the desk.” The speaker recalls leaving the key on the desk. 確か and はずだ do not turn that recollection into an inspection of the key’s current location.
Identify the object of confidence
Reliable delivery is a goal, whereas the key sentence concerns a recollection. Do not treat both as reports of a completed confirmation.
The certain arrival is also separate from the remembered key. Confidence in a recollection does not guarantee a current state, and uncertainty about success does not establish failure.
How do the purpose in 確実に届けるため and the recollection in 確か、鍵は机の上に置いたはずだ differ?
Show the answer and why
The first is a goal of reliably delivering the lost item; the second recalls where the key was left. Neither completed delivery nor the key’s current location is established.