表面看起來(lái)if not和but for似乎在意思上差不多,但是在詞性和用法上卻有很大的區(qū)別.
一、詞性：
* if not
1.副詞短語(yǔ),意思是“不然的話,否則” ,如：
We should do it with all our heart.If not we will leave it unfinished.
我們應(yīng)該全心全意地做這件事,不然的話,我們將會(huì)半途而廢.
2.連詞,引導(dǎo)否定的條件狀語(yǔ)從句,如：
If we are not healthy,happiness is impossible.
沒(méi)有健康就我們就不可能有幸福.
* but for 否定復(fù)合介詞,用在名詞前構(gòu)成介詞短語(yǔ),意思是“要不是”,等于 without.還可以擴(kuò)展成 if ...not / unless 從句（見(jiàn)下）.
二、用法：
* 連詞 if not 用于條件句可以用真實(shí)語(yǔ)氣,也可以用虛擬語(yǔ)氣,如：
1.真實(shí)語(yǔ)氣：
If he does not study hard,he will never pass the examination.
他如果不努力學(xué)習(xí),（將來(lái)）考試就永遠(yuǎn)不會(huì)及格.
This baby seldom cries if he is not tired.
這個(gè)嬰兒除非在疲倦時(shí)（往往）很少哭.
If I do not understand what he says,I always ask him.
要是（平時(shí)）我不明白他所說(shuō)的,我總是問(wèn)他.
2.虛擬語(yǔ)氣：
How could I find you here if you were not at home?
要是你不在家,我怎么會(huì)在這里找到你呢?（對(duì)現(xiàn)在虛擬）.
If he had not told me the truth about her,I would not have uderstood what she had said.
他假如沒(méi)有告訴過(guò)我她的真相,我就不會(huì)明白他所說(shuō)過(guò)的話.（對(duì)過(guò)去虛擬）
* 但是,句子中只要有 but for,該句子必須用虛擬語(yǔ)氣.
But for your help （if you had not helped us）we should not have finished in time.
要不是你的幫助,我們就不會(huì)及時(shí)完成.
But for the rain (= If it had not rained) we should have had a pleasant journey.
要不是下雨,我們就會(huì)有一個(gè)愉快的旅途的.