含蓄条件句， 它所在的句子用虚拟语气， 表示与过去的情况相反。含蓄条件句主要通过以下两种方式来表现， 句中的虚拟语气一般表示过去的情况：
一、under， without和but for等介词短语表现含蓄条件句：
1 We could have done better under more favourable conditions(=if we had been given more favourable conditions). 在更有利的条件下， 我们还可以做得更好些。
2 Without your help(= If you hadn＇t helped me)， I couldn＇t have suc－ceeded． 没有你的帮助， 我是不可能成功的。
3 But for the heavy traffic(= If the traffic had not been so heavy)， we would have arrived earlier． 要不是交通拥挤， 我们会早到一些的。
　　4 That would have been considered impossible in the past(= if it had been in the past). 这在过去会被认为是不可能的。
二、but引起的句子， 结合上下文以及其他方式表现含蓄条件句：
1 He would have given you more help， but he has been so busy(= if he had not been so busy). 他本来要多给你一些帮助的， 只是他太忙了。
2 I would have written before， but I have been ill(= if I had not been ill). 本来该早给你写信的， 但是我生病了。
3 I was ill that day． Otherwise I would have taken part in the parade． (= If I had not been ill that day， I would have taken part in the parade．) 我那天生病， 否则就会参加游行了。
4 Anybody in her position would have done the same．(= If anybody had been in her position， he / she would have done the same．) 任何人处于她的位置都会这样做。
5 It was so quiet and you could have heard a pin drop． 是那样的安静， 掉根针都听得见。
6 A less brave man wouldn＇t have dared to work in the enemy head－quarters． (= If a man had been less brave， he wouldn＇t have dared to work in the enemy headquarters.)一个不够勇敢的人是不敢在敌人的总部工作的。