這兩個複合從屬連詞的用法和意義完全一樣,但as if 在當代英語中比 as though 普通得多.
1）as if 引出的表語從句所表示的情況是事實或具有很大可能性時,通常用直陳語氣.名子的謂語常為look, seem, taste, smell, sound、appear等.如：
It looks as if they have succeeded in the experiment.看來好像他們試驗成功了.
It seems as if you are the first one here.
看來你是第一次來這兒。
注意這個句子裡的one 為什麼不是time，值得大家思考。本例句摘自趙振才《英語常見問題解答大詞典》P1252；
如果相不明白，最好久背下來這個例句。
It seems as if there will be an elect soon.看來很快就要舉行選舉了。
It seems as if the weather is improving.看來天氣好像要變好了。
當從句子的含義明顯看出從句所表達的內容是非真實的時候，使用陳述語氣和虛擬語氣是沒有什麼區別的，如：
He talks as if he has (had) a potato in his mouth.他說起話來就好好像嘴裡含著一個土豆似的。
2) as if 引出的方式狀語從句所表示的情況不是事實,而是主觀想象或誇大性的比喻時；或者從句所表達的內容完全與事實實際情況相反或純粹是一種假設時通常用虛擬語氣.從句中謂語動詞一般用過去時,be 動詞常用 were,但在現代英語中也常見到單數 was的情況.（如果出現在考試中用were保險些）如：
It seems as if I were there.好像我昨天在那一樣。
He spoke to me as if I were deaf.
他那樣給我說話，就像我是聾子似的。
she spoke as if she was ashamed.她講話好像害羞似的。
The young man with long hair looks as if he were a girl.這位滿頭長髮的男青年看上去像個女的。
The injured man acted as if nothing had happened to him.這位受傷的男子表現得好像沒發生什麼事一樣。
She treats me as if I was (were) a stranger.
她對待我的態度，就好像我是陌生人似的。
They sat and looked at him as though he was disturbing them.
他們坐著，瞧了瞧他，好像他打擾了他們似的。
He spoke so casually as though the matter was of little consequence.
他說的很隨便，好像這事兒無足輕重似的。
3）as if 引出的方式狀語從句,當主句謂語是過去時態時,從句謂語形式常常沒有什麼特殊之處.如：
The lady felt as if she lost something.這位女士好像學得丟了什麼東西.
He was singing as though / if he hadn't had / didn't have a care in the world.他唱著歌,好像人世間他沒有憂慮.
I felt as though I'd been lying in the sun for hours.我覺得好像已經曬了好幾個小時的太陽。
He spoke to me as if I were deaf.
他那樣給我說話，就像我是聾子似的。
They stared at me as if I was crazy.他們盯著我，就像看瘋子一樣。
The new students talked as though they had been friends for years.
新來的學生們說話很親熱，就像多年的老朋友似的。
She acted as though nothing had happened．
她裝得好像什麼事也沒發生過似的。
4）as if 引導的同一從句中所用的時態不同,意思往往不同.請比較：
* The clown looks as if he were ill.那個小丑好像害病似的.（虛擬語氣）
* The clown looked as if he had been ill.那個小丑過去好像生過病.（過去生過病或虛擬語氣）
下面從as if/though 引導句型角度來分析：
引導方式狀語從句
例：He spoke to me as if I were deaf.
他那樣給我說話，就像我是聾子似的。
They stared at me as if I was crazy.他們盯著我，就像看瘋子一樣。
The new students talked as though they had been friends for years.
新來的學生們說話很親熱，就像多年的老朋友似的。
He closed his eyes as though he were too tired．
他閉著眼睛，好像太累了。
She acted as though nothing had happened．
她裝得好像什麼事也沒發生過似的。
注意：當從句主語和主句主語一致，可以把主語和to be
一起省去，也就是說表面看來是直接引出了不定式、現在分詞、過去分詞、介詞結構。如：
He opened his mouth as if (he wanted ) to speak.他張開嘴，好像是想說話。
He shook his head as if (he wanted ) to say “no”.他搖了搖頭，好像是想要說“不”。
Parents were torn from their homes, husbands from wives, to disappear forever as if swallowed up in death.
父母從家裡被奪走，丈夫從妻子身邊被擄去，永遠地消失了，猶如被死亡吞沒一樣。
He looked about as though (he was)in search of something．
他四下張望，好像尋找什麼。
He paused as if(he was)expecting Tom to speak．
他停了下來好像期望Tom說些什麼。
He listened as though （he was） turning to stone．
這孩子一動不動地聽著，彷彿變成了石頭似的。
引導表語從句
She looked as if she'd had some bad news.她看上去好像有什麼壞消息似的。
It looks as if it’s going to rain．
看樣子天要下雨了。
It seems as if I"m the first one here．
看起來我是第一個到這裡的。
as though和as if從句用虛擬語氣，還是用陳述語氣。完全根據具體情況而定。如果
從句表示的意思與事實完全相反，或者純粹是一種假設，通常用虛擬語氣。
When a pencil is partly in a glass of water．it looks as if it were broken．
當鉛筆一部分放進水裡，看起來好像折斷了似的。(虛擬)
You look as if you are friends．
你們看起來像朋友似的。
如果從句所表達的內容被看作是事實或有可能是真實的，就要用陳述語氣：
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.
看上去她好像又墜入愛河似的。
It looks as though we shall have to do the work ourselves.
看來我們好像不得不自己來做這項工作了。
It seems as if we shall have to walk.
看來我們好像不得不走著去了
It look as if my parents treat me as a visitor and guest.（高考題）
我的父母好像把我當作訪問者和客人似的。
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