落ちる and 落とす: falling, losing, and removing stains
Start with something falling and something being dropped, then follow the verbs into everyday meanings involving lost items, failed tests, and laundry.
Fall from a source or drop something onto a surface
落ちる describes an object falling from a higher position or from where it was held. ポケットから鍵が落ちました。 marks the source with から and the falling key with が.
落とす describes an event that causes an object to fall. うっかり鍵を床に落としました。 can describe an accident, so 落とす does not always imply a deliberate action.
Lost belongings and unsuccessful tests
財布を落としました。 often means that someone dropped and lost a wallet. スマホを床に落とした specifies a surface onto which it fell, while スマホを電車で落とした can mean it was lost on the train.
Use 試験に落ちる for failing an exam or screening. 弟は運転免許の試験に落ちました。 marks the exam with に and does not describe a physical fall.
A stain comes out or someone removes it
When a stain disappears in the wash, say 洗濯したらシャツの汚れが落ちました。 The stain that came out takes が with 落ちる.
To say that a detergent can remove a stain, use この洗剤で油汚れを落とせます。 The stain that can be removed takes を, and 落とせる is the potential form of 落とす.
Quick check: In “The coffee stain came out in the wash,” should you say しみが落ちました or しみを落ちました?
Show the answer and why
Say コーヒーのしみが落ちました。 The sentence describes the stain disappearing, so use が with 落ちる. For the action of removing it, say しみを落としました。