我來回答這個(gè)問題吧,
z8_cn 說得不錯(cuò),has 是現(xiàn)在完成時(shí),表示一直延續(xù)到現(xiàn)在的動(dòng)作. had是過去完成時(shí),表示一直延續(xù)到過去某個(gè)時(shí)間點(diǎn)的動(dòng)作.Had完成時(shí)一般要與表時(shí)間的狀語(yǔ)連用.比如.
He had been waiting there for 3 hours when I got to the airport yesterday.
當(dāng)我昨天趕到機(jī)場(chǎng)的時(shí)候,他已經(jīng)在那里等了3小時(shí)了.等這個(gè)動(dòng)作一直延續(xù)到我昨天去機(jī)場(chǎng)這個(gè)時(shí)間點(diǎn),所以用過去完成時(shí),這里有時(shí)間狀語(yǔ)when I got to the airport yesterday.
在機(jī)場(chǎng)見面的當(dāng)時(shí),他可以這么說
Why do you come so late? I have been waiting here for 3 hours!
因?yàn)榫退f話的時(shí)候,等的動(dòng)作是延續(xù)到現(xiàn)在的.用現(xiàn)在完成時(shí).
kingofhill舉的第二個(gè)例子不是很完整,如果有上下文或在一個(gè)對(duì)話的環(huán)境中才可以這么說,過去完成時(shí)一般不會(huì)脫離時(shí)間狀語(yǔ)而獨(dú)立出現(xiàn)
--- What did you do when you were in US? (這時(shí)你已經(jīng)回國(guó)了)
--- I had been learning English for five years. (這里后面其實(shí)省略了before I came back to China, 在我回國(guó)之前,我在那里學(xué)了5年英語(yǔ).現(xiàn)在有可能還在學(xué)也有可能沒學(xué)了,但這個(gè)不重要,不是他們討論的重點(diǎn))
回到你問的這個(gè)句子,這里If Newton lived today 是一個(gè)虛擬條件從句,對(duì)后面主句的時(shí)間段沒影響
he would be surprised by what___ in science and technology 這個(gè)句子中,主語(yǔ)是what has discovered in science and technology, 這里的discover是延續(xù)到今的,至今為止所發(fā)現(xiàn)的科學(xué)技術(shù)
如果要用had discovered, 要這么說：
If Newton lived today, he would be surprised by what had discovered in science and technology before 21st century.
如果牛頓住還在世的話,他會(huì)為被21世紀(jì)前所發(fā)掘的科學(xué)和技術(shù)感到驚訝.
這里就是過去的一個(gè)時(shí)間點(diǎn)（2000年1月1日）之前的動(dòng)作的延續(xù).
希望你明白了