新概念英语二册55课第二句话中为什么要用现在完成时呢?

首先讲一下现在完成时的三种用法:
1.表示已完成的动作；2.表示未完成的动作或状态,可能继续下去,也可能刚刚结束；3.表示从过去到现在期间反复发生的动作或多次出现的状态.
第一句.A new machine called‘The Revealer’has been invented and it has been used to detect gold which has been buried in the ground.
a.本句中invent处属于用法1;use处属于用法3;bury处属于用法2.
b.3个has been的原理即是平行结构,又符合一种写作技巧repetition(重复).
第二句:The pirates would often bury gold in the cave and then fail to collect it.
因为过去时已经在would上体现了.bury and fail是一个并列的结构,要求两者形式一致.把此句展开的话可以理解为:
...would bury and would fail...
第三句:A new machine called‘The Revealer’has been invented and it has been used to detect gold which has been buried in the ground.
用完成时的原因已经在第一句中说了.
invent处其实也可以用过去式表达,而后两处用过去式显然表达不出"从过去到现在"一直处于的状态,会造成语意混乱现象.(美语中有用过去式替代现完的趋势,但仅限于口语,书面中很少见.)
因为后两个都用现完,所以invent也用现完,平行结构.