1. 由but for, with, otherwise等介詞短語(yǔ)引導(dǎo)的句中。此類(lèi)介詞短語(yǔ)多引導(dǎo)含條件意味的虛擬語(yǔ)氣。如： ①The ship would have sunk with all on board but for the efforts of the captain. 要沒(méi)有船長(zhǎng)的努力，那這只船及船上所有的人都沉沒(méi)了。 ②With your help, I would do the experiment well. 有了你的幫助，我會(huì)把實(shí)驗(yàn)好好地做的。 ③I am really busy, otherwise I would certainly go with you. 我真得好忙，否則我當(dāng)然跟你一起去了。 2. if only引導(dǎo)的特殊句式。此種句式相當(dāng)于感嘆句，表示“要是…… 就好了!”，表示現(xiàn)在的愿望用過(guò)去時(shí);表示過(guò)去的愿望用過(guò)去完成時(shí);表示將來(lái)的愿望用過(guò)去時(shí)或過(guò)去將來(lái)時(shí)。如： ①I(mǎi)f only I were a bird!要是我是一只鳥(niǎo)就好了。 ②If only we had listened to his advice!我們要是聽(tīng)了他的建議就好了。 ③If only it would stop raining!但愿雨能停下來(lái)!