if + 介詞短語

　　這類結構往往要根據(jù)具體的語境來理解，但有些經常搭配的慣用結構也值得注意，如if in doubt, if at all, if by any chance等。如：

　　If in doubt, ask your doctor. He can give you further information. 你若有疑問，可以問問醫(yī)生. 他會向你作進一步的說明。

　　Their policies have changed little, if at all, since the last election. 自上次選舉以來，他們的政策就算是有所變化，也變得很少。

　　If by any chance you cant manage dinner tonight, perhaps we can at least have a drink together. 就算你今晚不吃晚飯，也許我們至少可以一起喝一杯。