We walked across the street,across是介词还是副词,书上说是介词如果是介词,可walk是

across这里是介词,across the street 整个是介词词组,做状语,就可以修饰动词walk 了
告诉楼主判断介词副词小方法：
如果这个词后面跟有名词,就是介词.如：look around the room.这里 around就是介词.
如果后面没有名词了,就是副词了.如 he looked around.这里around就变成副词了.