留める and 止める: fastening and stopping with とめる
Compare fastening documents with stopping a car through representative combinations. Do not turn the kanji choice for the same reading into an absolute rule for every context.
Fasten documents with 留める
留める, read とめる, can mean fasten or keep in mind. 書類を金具で一つに留めた。 says that the documents were fastened together with a clip.
The focus is fastening the documents together. Distinguish this object and action from stopping the movement of a machine or car.
Stop a car with 止める
止める is read とめる here. 係員が手を上げて車を止めた。 says that the employee raised a hand and stopped the car.
Both raising the hand and stopping the car are stated. Compare these representative combinations without judging every context from the kanji alone.
Keep something in mind with 留める
旅行中に聞いた言葉を心に留めておく。 says that someone keeps or will keep in mind the words heard during the trip. 聞いた modifies words already heard; 留めておく is not a past completed form.
留める also has uses beyond physical fastening. Learn the representative combination and context rather than imposing mutually exclusive spellings on all fastening and stopping expressions.
Which representative combination is supplied for fastening documents together with a clip?
Show the answer and why
書類を金具で留める. Read the fastening focus without extending this example into a spelling rule for every use of とめる.