不定式作定语和作宾补的区别？

I need a pen to write with . 中 to write with 是pen的定语，不是补足语，如果是宾语补足语的话pen和to write with 有主谓关系,但很显然pen不可能去write,因为只有人才能去write.
His wish to buy a car came true 中的to buy a car 做his wish 的定语。