If Newton lived today,he would be surprised by what _____

我来回答这个问题吧,
z8_cn 说得不错,has 是现在完成时,表示一直延续到现在的动作. had是过去完成时,表示一直延续到过去某个时间点的动作.Had完成时一般要与表时间的状语连用.比如.
He had been waiting there for 3 hours when I got to the airport yesterday.
当我昨天赶到机场的时候,他已经在那里等了3小时了.等这个动作一直延续到我昨天去机场这个时间点,所以用过去完成时,这里有时间状语when I got to the airport yesterday.
在机场见面的当时,他可以这么说
Why do you come so late? I have been waiting here for 3 hours!
因为就他说话的时候,等的动作是延续到现在的.用现在完成时.
kingofhill举的第二个例子不是很完整,如果有上下文或在一个对话的环境中才可以这么说,过去完成时一般不会脱离时间状语而独立出现
--- What did you do when you were in US? (这时你已经回国了)
--- I had been learning English for five years. (这里后面其实省略了before I came back to China, 在我回国之前,我在那里学了5年英语.现在有可能还在学也有可能没学了,但这个不重要,不是他们讨论的重点)
回到你问的这个句子,这里If Newton lived today 是一个虚拟条件从句,对后面主句的时间段没影响
he would be surprised by what___ in science and technology 这个句子中,主语是what has discovered in science and technology, 这里的discover是延续到今的,至今为止所发现的科学技术
如果要用had discovered, 要这么说：
If Newton lived today, he would be surprised by what had discovered in science and technology before 21st century.
如果牛顿住还在世的话,他会为被21世纪前所发掘的科学和技术感到惊讶.
这里就是过去的一个时间点（2000年1月1日）之前的动作的延续.
希望你明白了