if + 介词短语
这类结构往往要根据具体的语境来理解，但有些经常搭配的惯用结构也值得注意，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. 你若有疑问，可以问问医生. 他会向你作进一步的说明。
Their policies have changed little, if at all, since the last election. 自上次选举以来，他们的政策就算是有所变化，也变得很少。
If by any chance you cant manage dinner tonight, perhaps we can at least have a drink together. 就算你今晚不吃晚饭，也许我们至少可以一起喝一杯。