虛擬句這次只探討對過去情況的虛擬句，其句型為would/could/should/might have done。
區(qū)別現(xiàn)在完成時(shí)里所發(fā)生的事情都是真實(shí)的，并且時(shí)間點(diǎn)在現(xiàn)在。而虛擬句指不能實(shí)現(xiàn)或純假想的情況，可以對過去、現(xiàn)在或?qū)磉M(jìn)行假想。虛擬語氣的本質(zhì)就是對根本不可能的情況發(fā)生假設(shè)。
例句對比（1）I have caught the bus because you helped me. 因?yàn)槟愕膸椭亿s上了車。If it had not been for your help yesterday, I could not have caught the bus. 如果昨天沒有你的幫助，我不會(huì)趕上公交車。（事實(shí)是你昨天已經(jīng)幫助了我，所以我趕上了車)
（2）We have done something. 我們做了些什么。If we had realized that we would have done differently. 如果我們早意識(shí)到這一點(diǎn)的話，我們的做法就會(huì)完全不同。（事實(shí)上沒及時(shí)意識(shí)到。）
（4）He has been away from the city. 他已離開這個(gè)城市。If he had known the news, he would have stayed here. 如果他知道了那個(gè)新聞，他便會(huì)留下來。（事實(shí)上他不知道那個(gè)新聞，沒留下來）
（5）He hasn't finished his work. 他沒把工作做完。If he had realized the importance of the work, he would have finished his work. 如果他意識(shí)到了這個(gè)工作的重要性，他就會(huì)把工作做完了。（事實(shí)上沒意識(shí)到，沒做完。）
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