as if和as though有什么区别,什么时候用前者什么时候用后者?

as if,as though的用法：
这两个复合从属连词的用法和意义完全一样,但as if 在当代英语中比 as though 普通得多.
1）as if 引出的方式状语从句所表示的情况是事实或具有很大可能性时,通常用直陈语气.名子的谓语我为look,seem,taste,smell,sound等.如：
It looks as if they have succeeded in the experiment.看来好像他们试验成功了.
2) as if 引出的方式状语从句所表示的情况不是事实,而是主观想象或夸大性的比喻时,通常用虚拟语气.从句中谓语动词一般用过去时,be 动词常用 were,但在现代英语中也常见到单数 was的情况.如：
The young man with long hair looks as if he were a girl.这位满头长发的男青年看上去像个女的.
The injured man acted as if nothing had happened to him.这位受伤的男子表现得好像没发生什么事一样.
3）as if 引出的方式状语从句,当主句谓语是过去时态时,从句谓语形式常常没有什么特殊之处.如：
The lady felt as if she lost something.这位女士好像学得丢了什么东西.
He was singing as though / if he hadn't had / didn't have a care in the world.他唱着歌,好像人世间他没有忧虑.
4）as if 引导的同一从句中所用的时态不同,意思往往不同.请比较：
* The clown looks as if he were ill.那个小丑好像害病似的.（虚拟语气）
* The clown looked as if he had been ill.那个小丑过去好像生过病.（过去生过病或虚拟语气）