only if表示"只有"；if only則表示"如果……就好了".If only也可用于陳述語氣.
I wake up only if the alarm clock rings. 只有鬧鐘響了,我才會醒.
If only the alarm clock had rung. 當(dāng)時鬧鐘響了,就好了.
If only he comes early. 但愿他早點(diǎn)回來.
辨析only if 與 if only

only與if這兩個單詞隨意組合,可以組成兩個意義相近,但用法不盡相同的短語only if與if only.現(xiàn)將它們的具體用法辨析如下：
一、only if意為“only on the condition that, as long as”,“只要；假如”.它用于陳述事實(shí)或必要的條件,比if所表達(dá)的語氣更強(qiáng)烈.
You'll pass this course only if you study. 只要你學(xué),你就會通過這門課.
Children may enter the club only if they are accompanied by adults. 只要孩子們有大人陪同就可以參加俱樂部.
The students at the university can take up part-time job only if it doesn't interfere with their studies. 只要不影響學(xué)業(yè),這所大學(xué)的學(xué)生可以從事兼職工作.
二、if only則有兩種含義.第一種含義與only if近似,意為“as long as”,“只要”.例如：
If only it clears up, we'll go. 只要天一放晴,我們就去.
第二種則是“expressing a wish or assumption”, “但愿；如果……就好了”.它用來表達(dá)一個不可能實(shí)現(xiàn)的愿望或假設(shè),句子要用虛擬語氣.例如：
If only she would come!要是她能來就好了!
If only you had told me you had a spare ticket for the show. 如果你告訴我,你多了一張演出票就好了.
If only you had worked with greater care!你要是工作更仔細(xì)一些該多好!