因为是"在纸上写",而不是"写纸"正确:write on a piece of paper 在一张纸上写错误:write a piece of paper 写一张纸纸所以当用不定式作定语修饰paper时,就必须用write on.类似的句子:He needs a room to live in .Do...嗯，我再看看，再没有更好的就选你了TVT不定式作定语修饰名词时, 当不定式的动词与被修饰的名词不是动宾关系时, 动词后就必须加介词. 如: I use a pen to write with.注意这个with不能少（“用笔写字”是write with a pen）There is nothing （for us）to think about.（“考虑某事”是think about sth）