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 引出的表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.名子的谓语常为look, seem, taste, smell, sound、appear等.如：
It looks as if they have succeeded in the experiment.看来好像他们试验成功了.
It seems as if you are the first one here.
看来你是第一次来这儿。
注意这个句子里的one 为什么不是time，值得大家思考。本例句摘自赵振才《英语常见问题解答大词典》P1252；
如果相不明白，最好久背下来这个例句。
It seems as if there will be an elect soon.看来很快就要举行选举了。
It seems as if the weather is improving.看来天气好像要变好了。
当从句子的含义明显看出从句所表达的内容是非真实的时候，使用陈述语气和虚拟语气是没有什么区别的，如：
He talks as if he has (had) a potato in his mouth.他说起话来就好好像嘴里含着一个土豆似的。
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；或者从句所表达的内容完全与事实实际情况相反或纯粹是一种假设时通常用虚拟语气.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.（如果出现在考试中用were保险些）如：
It seems as if I were there.好像我昨天在那一样。
He spoke to me as if I were deaf.
他那样给我说话，就像我是聋子似的。
she spoke as if she was ashamed.她讲话好像害羞似的。
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。
She treats me as if I was (were) a stranger.
她对待我的态度，就好像我是陌生人似的。
They sat and looked at him as though he was disturbing them.
他们坐着，瞧了瞧他，好像他打扰了他们似的。
He spoke so casually as though the matter was of little consequence.
他说的很随便，好像这事儿无足轻重似的。
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.
I felt as though I'd been lying in the sun for hours.我觉得好像已经晒了好几个小时的太阳。
He spoke to me as if I were deaf.
他那样给我说话，就像我是聋子似的。
They stared at me as if I was crazy.他们盯着我，就像看疯子一样。
The new students talked as though they had been friends for years.
新来的学生们说话很亲热，就像多年的老朋友似的。
She acted as though nothing had happened．
她装得好像什么事也没发生过似的。
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）
下面从as if/though 引导句型角度来分析：
引导方式状语从句
例：He spoke to me as if I were deaf.
他那样给我说话，就像我是聋子似的。
They stared at me as if I was crazy.他们盯着我，就像看疯子一样。
The new students talked as though they had been friends for years.
新来的学生们说话很亲热，就像多年的老朋友似的。
He closed his eyes as though he were too tired．
他闭着眼睛，好像太累了。
She acted as though nothing had happened．
她装得好像什么事也没发生过似的。
注意：当从句主语和主句主语一致，可以把主语和to be
一起省去，也就是说表面看来是直接引出了不定式、现在分词、过去分词、介词结构。如：
He opened his mouth as if (he wanted ) to speak.他张开嘴，好像是想说话。
He shook his head as if (he wanted ) to say “no”.他摇了摇头，好像是想要说“不”。
Parents were torn from their homes, husbands from wives, to disappear forever as if swallowed up in death.
父母从家里被夺走，丈夫从妻子身边被掳去，永远地消失了，犹如被死亡吞没一样。
He looked about as though (he was)in search of something．
他四下张望，好像寻找什么。
He paused as if(he was)expecting Tom to speak．
他停了下来好像期望Tom说些什么。
He listened as though （he was） turning to stone．
这孩子一动不动地听着，仿佛变成了石头似的。
引导表语从句
She looked as if she'd had some bad news.她看上去好像有什么坏消息似的。
It looks as if it’s going to rain．
看样子天要下雨了。
It seems as if I"m the first one here．
看起来我是第一个到这里的。
as though和as if从句用虚拟语气，还是用陈述语气。完全根据具体情况而定。如果
从句表示的意思与事实完全相反，或者纯粹是一种假设，通常用虚拟语气。
When a pencil is partly in a glass of water．it looks as if it were broken．
当铅笔一部分放进水里，看起来好像折断了似的。(虚拟)
You look as if you are friends．
你们看起来像朋友似的。
如果从句所表达的内容被看作是事实或有可能是真实的，就要用陈述语气：
He walks as if he is drunk.
他走路好像喝醉了似的。
It looks as if she is in love again.
看上去她好像又坠入爱河似的。
It looks as though we shall have to do the work ourselves.
看来我们好像不得不自己来做这项工作了。
It seems as if we shall have to walk.
看来我们好像不得不走着去了
It look as if my parents treat me as a visitor and guest.（高考题）
我的父母好像把我当作访问者和客人似的。
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