要說：The pen is used for writing with. 是因為要說：write with a pen　　

要說：The pen is used to write with. 是因為要說：write with a pen　　

要說：Give me a piece of paper to write on. 是因為要說：write on a piece of paper　　

要說：Give me a pen to write with. 是因為要說：write with a pen