“用书面写”常见的英文表达可以是 “write it down in writing” 或更简洁的 “write it in written form” ，也可表述为 “put it in writing” 。
write it down in writing：强调将内容以书面形式记录下来， “write it down”有“写下”的意思，“in writing”进一步明确是书面形式。例如：Please write it down in writing so that we can refer to it later.（请用书面形式写下来，以便我们日后参考。）
write it in written form：直接表达“以书面形式写它” ， “written form” 即书面形式。例如：You should write your complaint in written form.（你应该把你的投诉用书面形式写下来。）
put it in writing：这是一个比较常用的短语，意思是“以书面形式确定下来；用书面写下来” 。例如：I think we should put this agreement in writing.（我觉得我们应该把这个协议用书面形式确定下来。 ）