Could you lend me a pen to write with?为什么要加with

Could you lend me a pen to write with?为什么要加with Will you lend me a pen to write with,为什么要用with? Will you lend me a pen to write with?最后的介词问什么用with?语法点是什么? The pen is good to write为什么要加with （I have a pen to write with）句中 为什么加 with can you write this letter for me with a pen? I want to borrow a pen ____ to write with 为什么要用with I'm sorry to trouble you .could you lend me a pen ,please.Of I use a pen to write.还是I use a pen to write with.为什么? Do you have a pen to write with? please pass me a pen ___.A.to write on B.write with C.to wri write with a pen