He described where they had landed and told how they were moving across the USA
问题描述：
He described where they had landed and told how they were moving across the USA
为什么前面是过去完成时,而后面是过去进行时they were moving across
答
had landed 是相对谓语described 来说的,由于先于described 发生,过去的过去,所以是过去完成时
were moving 侧重陈述过去的某一特定时候的情景,状态,所以用过去进行时
简单点说.用过去完成时,表示在过去某一特定事件以前做过某事
用过去进行时则表示在过去某一特定时间在做的事
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