I have a pen to write with.后边为什么要加个with?有什么规律?

I want to writ a novel i have a pen to write还是/write with 判断：I have a pen.____ I have pen.____ can you call me at 7:30pm,I wait you sorry I have to writ letter in english , I have no pen ______ .May I borrow yours?A.to write B.to write with I have a good pen. 怎样强调a good pen? Do you have a pen I want to ( ) a letter to my parents I have a pen ,haven't I 还是I have a pen ,don't I I have a pen 变否定句 I have a new pen friend i ( )have a pen friend in england. 英语翻译I want to have a pen friend,hopefully a girl in her early twenties,and... I didn't have any money to buy a pen.(改为同义句） I ____ ____ money to buy a pen. I have a new pen pal in Australia.We often （ ）to each other. （I have a pen to write with）句中 为什么加 with I am happy to have a new pen pal.中文意思 happy,i,am,to,new,a,pen pal,have (连词成句)只有五分钟 I have a pen to write with.后边为什么要加个with?有什么规律?