什么时候用write?什么时候用write with?

i have a pen to ------write with
i have a letter to ------write
你可以把句子还原.你是要用笔来写,而不是要写笔,对吧.
i write with a pen.
你是要写信而不是要用信写,对吧.
i write a letter.
i want to find a hotel to stay in
还原成 i stay in a hotel.
i want to find somewhere to stay
这里的somewhere 是个副词,可以直接加动词后面.
还原成i stay somewhere